The effects of constant soil moisture levels of 90, 60 and 40% of the maximum capillary capacity, applied beginning from the planting of the germinated kernels, on the dynamics of the foliage development of maize was studied in relation to ontogenical changes in leaf area of individual leaves. There were two maxima in the growth of total leaf area (characterized by leaf area duration and loaf area relative growth rate) unrelated to the soil mositure. The first maximum which was less marked, appeared in the phase of 5-8 leaves, the second and main increase being observed before flowering in the phase of 10-12 leaves. The effects of continued decrease in soil moisture were a correspondingly progressive reduction in leaf area and delayed development of the whole plants. Plants with 60% soil moisture attained the same leaf area as those with 90% but only after the end of the vegetative phase. The greatest differences in rate of development between watering treatments appeared at the time of greatest increase in leaf area of control plants. Continued decrease in soil moisture markedly affected the development of the leaves at different insertion levels (especially in elder leaves). Clear-cut prolongation of ontogenesis took place under dry conditions. In conditions of higher soil moisture growth was rather limited or stopped after reaching a certain maximum.

A virus having 300 nm long rod-shaped particles was isolated fromPlantago media L. in Yugoslavia. The virus was transmitted to 15 species of host plants the symptoms of which are described in detail. The symptoms corresponded to those that appeared after infection by the original Holmes' ribgrass virus (HRV).
The investigated virus was compared both with the common strain of tobacco mosaic and the original Holmes' ribgrass viruses by means of serological tests. The agar double-diffusion tests showed that it is closely related to HRV and remotely related to the common strain of TMV. On the basis of these results we concluded that this virus represented a strain of the HRV.
The investigations of the cell inclusions showed that our virus produced rounded plates instead of hexagonal prisms. Electron micrographs of ultrathin sectioned material demonstrated that these plates were formed by virus particles lying perpendicularly to the layers of the plates. The presence of plates also points to the fact that the investigated virus belongs to the HRV.

The following work deals with the role of auxin in growth and differentiation processes of isolated shoot tips ofZea mays L. It seems that an optimal auxin concentration is apparently a function of a photoperiod duration. In case of a short day the optimal concentration is approximately 5 mg/l and in case of a long day it represents 1 mg/l. The duration of photoperiod influences even the level of exogenous auxins in explants. The plants exposed to the short photoperiod contain more substances of stimulative character while the ones exposed to the long day possess inhibitive substances. Externally applied auxin takes part in differentiation processes, especially in the formation of female elements in tassel spikelets and in transformation of the tassel primordium in pistillate ear.

The effect of Ca-deficiency on changes in the protein content of cotyledons and roots of germinating pumpkin (Cucurbita pepo L.) plants was studied by means of immunoelectrophoresis. Antiserum against seed proteins gave 11 clear precipitation zones in homological reactions and 7 zones with proteins from cotyledons of 10 day-old plants. Several less mobile zones in the area of start appeared in variant "-Ca". Using antiserum against root proteins of 10 day-old control plants, 8 precipitation zones in the homological reaction and 6 zones (two cathodic zones are missing) in the heterological reaction were obtained. The patterns of proteins from the root and those arising from the protein of seed are both changed under Ca-deficiency.

A water-soluble germination- and growth-inhibitor present in plants of Brassica genus was studied. Aqueous eluates from chromatograms of methanolic extracts of Savoy cabbage, cabbage and Brussels' sprouts leaves, and of Brussels' sprouts roots, inhibited the growth of wheat; the chromatographic behaviour of this inhibitor was similar to that of the thioglycoside glucobrassicin (GLUBR). GLUBRat 10^-3 m, and the supposed product of its enzymatic degradation, 3-indolylacetonitrile (IAN) at 10^-4 m, inhibited the growth of wheat and clover in a similar manner. At lower concentrations, both substances gradually enhanced growth. In contrast to IAN, GLUBR did not appreciably influence the growth of Savoy cabbage. Other products of GLUBR degradation, such as thiocyanate ion and ascorbigen, influenced the growth to a lesser extent and in a different way. At higher concentrations GLUBR and IAN also inhibited the germination of clover seeds. The biological effect of GLUBR is dependent on the presence of the indole nucleus in its molecule; the thioglycoside glucoiberin, which is not of indolic character, does not influence growth. The presence of free myrosinase was shown in intact wheat seedlings. Hence, GLUBR taken in by the germinating plants must undergo enzymatic cleavage. The in vitro activity of myrosinase in wheat and clover was lower than in Savoy cabbage, but the quantity of IAN formed at a pH corresponding to the natural pH was higher in wheat and clover than in Savoy cabbage, where the formation of the growth-inactive ascorbigen prevailed. On the basis of these results the hypothesis was formulated that GLUBR, the indolic complex from the generaBrassica, Raphanus, Cochleria, Nasturtium, Barbarea andSisymbrium, is a representative of a class of potential inhibitors of growth and germination, whose activity can be explained by the liberation of large amounts of growth-affecting substances during their specific enzymatic cleavage.

The interactions between cereals-wheat, barley, rye and oats-in combined cultivation in mixtures of always two species in relation 1:1 in the initial phase of growth were studied. During thirty days' cultivation in pots the growth of the experimental plants in pure cultures and in mixtures and the changes of the nitrate nitrogen content of the soil in the experimental vessels were followed.
The experiments showed that all tested species of cereals interacted with each other during the growth in mixture. The growth changes began soon after sowing, were of a stimulating and inhibiting character and increased during the growth. An exception was the small reaction of oats to rye.
Decrease of the nitrate nitrogen content was determined in the soil of the experimental vessels during the cultivation of the plants. The changes of the nitrate nitrogen in the soil corresponded on the whole with the exception of the last phases of the experiments to the growth intensity of the experimental plants. No essential differences, either in the content or in the rate of the decrease of nitrate nitrogen, were found in the soil of control plants and in the soil of plant mixtures. Small differences which were manifested cannot be considered the primary cause of the mutual relations of cereals. The cause of the mutual relations may be attributed to allelopathic factors.

Samples of the leaf tissue (14cm²) were placed in a plexiglass chamber which consisted of three parts. Water absorbed by the leaf tissue on one side of the sample was transported through the middle part of the sample to the opposite side and was transpirated there. The intensity of transpiration the intensity of water absorption and water saturation deficit (w.s.d.) were determined simultaneously in this tissue by gravimetry. Water balance was studied either in saturated samples of leaf tissue or in tissue where w.s.d. (10%, 20%, 30%, 40%) was established in advance. Although conditions for water absorption in leaf segments were optimal, w.s.d. originated in the saturated leaf tissue under all given external conditions (evaporation from 41.7 to 17.8 mg cm^-2 h^-1). W.s.d. which was established in advance for the most part increased during the experiment and reached even high values (more than 60%). the equilibration was reached only under conditions of low evaporation and initial w.s.d. higher than 20% in young leaves and higher than 30% in adult leaves. A positive correlation between the ratio of the intensity of water absorption to the intensity of transpiration and w.s.d. was found only under conditions of lower evaporation (17.8 and 23.2mg cm^-2h^-1). The maximal values of w.s.d. were limited in this way.
Water balance was studied: 1. in leaf tissue of upper, middle and lower leaves of fodder cabbage, 2. in leaf tissue of middle leaves of young and adult plants of fodder cabbage, 3. in leaf tissue of dicots (fodder cabbage) with different vessel orientation in respect to water transport, 4. in leaf tissue of monocots (banana-tree) with water transport upright to the vessel orientation.
Considerable change of water balance was observed when the water transport was prolonged by two incisions in the middle part of the sample.
Results of all these experiments revealed the possibility of water stress origin even in leaf tissue sufficiently supplied with water.

A new mathematical model, representing the transpiration process during ontogenesis and a circadian transpiration cycle, analysed in detail statistically, was designed using data obtained by the new modification of the gravimetric determination of the transpiration of intact plants. Regression analysis applied to the circadian cycle makes it possible to characterize basis parameters of the intensity and of the extent of this process and may serve as a starting point when studying transpiration methodologically in greater detail.

In the pea test a highly positive response to the treatment with IAA reversed to a negative one or became 5 to 6 times weaker when CCC was applied together with IAA. In cultivating pea seedlings, following their decapitation, for two days in a 0.25 per cent CCC solution and then in water, growth of their cotyledonous axillaries (cotylaries) were inhibited. This inhibitive action of CCC could be made ineffective when the seedlings, following two-days' cultivation in the CCC solution, were grown further in kinetin solutions (0.37-3 mg per 1). Cotylaries of decapitated pea seedlings, when grown in kinetin solutions were inhibited. With kinetin solutions of 6-12 mg/l a strong inhibition also occured in the growth of roots at the apical parts of which spherical swellings were developing.
The CCC supplied to the roots of intact etiolated pea seedlings is translocated acropetally into the stem at a rate of about 5 cm per hour. Decapitation of the plant causes retardation of this transport, yet a coat of 0.00001-1% IAA or kinetin paste produces acceleration of the stream.
Existence of an antagonism between CCC and IAA, demonstrated earlier, was found holding true also for B-9 (N, N-dimethyl-aminesuccinamic acid) and IAA, as the inhibitive action of B-9, 0.06% solution on the growth of lettuce hypocotyls was reduced to a highly significant degree when the plants were supplied with B-9 together with IAA at a concentration of 10 mg/l.

In the experiment with 18 winter varieties the genotypic, phenotypic and environmental correlations were estimated in all combinations between the pairs of 11 quantitative characters.
Both yield of grain with its components and further characters were evaluated, of which flag leaf size and leaf angle of the last but one leaf are of importance for the utilization of light energy and consequently for receiving maximum rates of photosynthesis.
In simple genotypic and phenotypic correlations their different stability with regard to mineral nutrition, eventually to the date of sowing was proved.
In the discussion the limited signification of simple correlation for the statement of complicated processes in plants caused by multiplied associations of characters was pointed out. In the example of yield components of the plant at the application of 60 kg N per hectare it was shown that also further unknown variable factors might participate in the genotypic correlation. It may be concluded that the inclusion of them when constructing the selection indexes will give further foundations for handling yield as a breeding or genetic character, and for the causal analysis of yield formation in wheat.

Experiments were conducted to compare the efficiency of transmission of a strain of pea mosaic virus (PMV) isolated in Czechoslovakia by two strains (clones) of the pea aphidAcyrthosiphon pisum Harris (green and red) and one strain ofMyzus persicae Sulz. PMV is a nonpersistent virus and the preliminary fasting of aphids before acquisition feeding increases the efficiency of aphids in transmission of this virus. In our experiments two hour fasted individuals were used and two periods of acquisition feeding on the source (1 and 5 minutes). On the healthy test plants the aphids were left over night. As the source and test plants pea (Pisum sativum L.) of the cv. Raman were used. During the one minute acquisition period on the source of infection the aphids were observed under the stereoscopic microscope. They usually made two to three probes. During the five minute acquisition feeding time the aphids were not observed and they were taken from the source of infection after a lapse of five minutes. To compare the efficiency in transmission of this virus by these aphids only one aphid per tested plant was used and in all trials only two to four day old nymphs were taken. Differences in transmission efficiency between two strains ofAcyrthosiphon pisum Harris were highly significant. The green strain of pea aphid was the less efficient vector in comparison with the green peach aphid and the red strain of the pea aphid, the latter being the most efficient vector of this virus.

Potato leafroll virus induces considerable disturbances of phosphorus metabolism in leafroll-infected potato plants. Each variety reacts differently in this respect on the infection, to some extent being dependent on the susceptibility of the respective variety to leafroll and to the season as well. In varieties particularly susceptible to leafroll as in Apta and Sieglinde there is a decrease of total P after infection. On the contrary, varieties showing medium susceptibility such as varieties Tatranka, Rita and Ambra react to leafroll infection with a considerable increase of total P. In the comparatively less susceptible variety Krasava, there is no change in the level of total P after infection. In autumn, however, a slight decrease of total P even in the medium susceptible variety Tatranka, and the less susceptible variety, Krasava, can be found. Our results shed a new light on existing controversies in literature regarding phosphorus content in leafroll-infected potato plants.
When the changes in the contents of various phosphorus fractions in the infected plants are compared we can observe a tendency of lowmolecular fractions to rise. Among the different phosphorus fractions one common feature could be seen, except for the hypersensitive resistant variety Apta, namely, that the acid-soluble organic phosphates in the stems of all varieties examined, were considerably higher than in healthy stems.

In young pumpkin plants, the phosphorus was found not to be affected substantially by potassium deficiency whereas the deficiency of calcium has marked effects. Potassium distribution is not affected substantially by either phosphorus or calcium deficiency.
In short-term experiments, the presence of calcium affects markedly the total uptake of potassium but the potassium distribution remains unaltered. The stimulation of potassium uptake is far more pronounced in plants pre-cultivated in a complete nutrient solution than in those pre-cultivated in a calcium-deficient one; this is apparently due to profound metabolic changes taking place in plants grown without calcium.
The reserves of phosphorus and potassium stored in the cotyledons are easily mobilized and translocated so that they can be used for growth in early stages of development; the appearance of pathological symptoms of phosphorus or potassium deficiency is thus prevented.

The Euonymus Mosaic Virus was isolated fromEuonymus europaea. The virus was transmitted by sap to cucumber and other test plants, by seeds orAphis euonymi F., and it was soilborne, too. The sap from cucumbers lost its infectivity after 10 minutes at 80°C and after 35 days at 20-22°C. It is probably a new virus so far not described. The investigations of this problem are not finished, yet.

Several methods of NA isolation and estimation were examined in order to determine RNA in the whole root systems of young wheat plants. It was found out that during the hydrolysis of roots with perchloric acid according toOgur andRosen (1950),Spirin (1958) andHeitefuss (1965) a considerable amount of orcine-positive compounds is released which cannot be adequate to the RNA content. Therefore the separate RNA determination in the presence of DNA was excluded even after the NA fractionation by hydrolysis at various temperature and perchloric acid concentration. Besides NA hydrolysates contained a high amount of other compounds absorbing in the UV-region. Compounds interfering with both these methods were present especially in the basal parts of roots.
Using the method ofKern (1960) a nucleoprotein fraction was isolated containing about 75 per cent of the total NA content in the tissue. This fraction consisted of more than 90 per cent RNA combined with proteins, namely the ribosomal RNA. After the hydrolysis it could be estimated spectrophotometrically for the hydrolysate did not contain other compounds absorbing in the UV-region. In the root tips the content of this fraction was high in comparison with the basal parts of roots and it changed with the kind of nutrition.

Using paper chromatography and conventional methods of isolation, natural growth inhibitors were isolated from green leaves of different plants (Brassica oleracea, Zea mays, Pisum sativum andSalix rubra). All isolated inhibitors were found to be phenolic compounds and the chemical structure of most of them was determined; only the final structure of theBrassica inhibitor has not yet been ascertained. 500 mg of natural inhibitor ofPisum sativum was isolated from 1500 g of leaves and was identified as quercetin-glucosil-p-coumarate (QGC), described earlier byFuruya, Galston andStowe (1961). The structure of the natural inhibitor ofZea mays (4 mg from 100 g of leaves) was identical with p-coumaric acid and the chemical nature of the plant growth inhibitor fromSalix rubra (700 mg from 1,5 kg of leaves and young bark) was that of 2-chalconaringenin-glucoside or isosalipurposide, described earlier byCharaux andRabaté (1931) andHarborne (1966). All isolated substances had inhibiting properties in the straight growth test of wheat coleoptile sections and decreased the growth of isolated stem sections prepared from plants-donors of inhibitors. Thus, maximum growth inhibition (LG max) was attained, if wheat coleoptile sections were incubated with:Brassica inhibitor in the concentration of 0.5 mg/ml, withPisum inhibitor (QGC) in the concentration of 16 mg/ml, withZea inhibitor (p-coumaric acid)-0.35 mg/ml and with Salix inhibitor (isosalipurposide) in the concentration of 0.5 mg/ml. In small concentrations no mentioned substances were able to enhance the growth as actively as indolic auxins (on 250-300%); only slight growth activation in biotests was sometimes observed for low concentrations. Inhibition in p-coumaric acid was much more active in a free form than in the bound form as an acyl-rest of QGC. As a rule, the wheat coleoptile test was much more sensitive (3-5 times) to the plant growth inhibitors, than tests prepared from tissue and organs of plants-donors. The retardation activity of plant growth inhibitors is not correlated with their molecular weight. Dormin (or±abscissin II) was also tested on wheat coleoptile sections. In neither of the applied concentrations (10-0.05 μg/l range) was dormin able to depress straight growth of wheat coleoptile sections, but even in a 1.7 μg/l concentration it inhibited the IAA-activated growth of sections. However, additional experiments showed that dormin in higher concentrations (40 μg/l and more was able even to depress endogenous straight growth of wheat coleoptise sections. The differences between the properties of natural phenolic growth inhibitors and dormin were discussed.

In estimating the water-holding power and thus also their drought resistance, the portion of curvature as shown by the desiccation curves (transpiration, loss curves) appears most suitable in which the water-binding forces within a plant body feature a steep and characteristic rise. Since this portion of the curving stage coincides, in the majority of cases, also with the linear stage for the desiccation curves as described byCetl, this author's method of establishing drought resistance of plants (Cetl, 1953, 1957) may be applied to good advantage also to the woody species seedlings. For particularly urgent instances serving the purpose of orientation a modification of this method has been suggested, as follows: following cutting off the seedling the output of water is weighed at these intervals only: 0, 60, 120, and 240 minutes; as the criterion for drought sistance the resulting value for the rate of water output (v) is taken as measured between the interval of 60 and 120 minutes. When a detailed analysis is made it is advisable to consider the v value measured for the interval between 0 and 60 minutes and for that between 120 and 240 minutes. Moreover, it is likewise advisable to maintain the soil with experimental seedlings at the level of optimum moisture (60 to 80 per cent maximum capillary water capacity), for some 18 to 22 hours prior to the seedling's abscission.

Experimental investigations on the course of yellows-type virus infections in plantsTrifolium repens L. gave the following results:
During 24 and 48 hours after the completion of the test feeding period the clover phyllody and clover dwarf viruses spread in the plant maximally up to 5 cm. from the point of inoculation.
In the infected plants, the reproduced clover phyllody and clover dwarf viruses appeared on the average 5-6 days earlier than disease symptoms.
Acquisition feeding of leafhoppers onT. repens plants infected with the clover dwarf virus for different periods of time, revealed a decline in the virus concentration in the plants after 45 days from the appearance of disease symptoms. In plants infected with the parastolbur virus, this decline was not found.

Transpiration rates of leaf blades of irrigated and not irrigated spring wheat plants were studied in relation to the water content and growth changes in the test plants during their development. The applied irrigation stimulated the growth and slightly delayed the development of the test plants. It increased chiefly the water content and to a lesser degree the dry solid weight in the plant body. The quantitative and qualitative properties of the water content in the plant affected not only transpiration rates, but also the development of new and the dying off of old organs and tissues, especially of leaf blades. Transpiration rates in irrigated plants were markedly higher than in not irrigated plants. Mean transpiration rates of different leaf blades varied and were typical for each leaf blade. From the static aspect it was possible to express and even to explain some of the relationships and the heterogenity of the leaf blades on the same stem by the "Law of Zalensky". This involved mainly the mean values of growth characteristics and the investigated features of the water regime. On the other hand, from the dynamic viewpoint it was possible to divide the different leaf blades according to their transpiration changes into two groups. The first group includes the blades of the first to third leaf, the second group the blades of the fourth to sixth leaf and the ear. The capacity to control the water regime in the different blades is greatest at the stage of tillering, shooting and milk ripeness. During these developmental stages the marked decrease in transpiration, caused in the first place by a number of internal and not only external factors, was explained.

Identification trials were carried out to determine what virus causes a mosaic disease of sweet peas in Czechoslovakia. The found properties of the identified sweet pea mosaic virus, the character of its transmission and its host range prove that sweet peas in Czechoslovakia are attacked by common pea mosaic virus (CPMV). Some insignificant differences in properties between our virus isolate and CPMV were observed in the course of the determination of properties of our virus isolate on various host plants. The possible existence of more strains of CPMV is discussed. The transmission of sweet pea mosaic to the plants ofPhaseolus vulgaris L. was negative. For this reason bean yellow mosaic virus (BYMV) was eliminated as a possible pathogen of our virus isolate. At the same time the indicator plantsChenopodium giganteum Don. andChenopodium guinoa Willd. with eight developed leaves were established to be most suitable for the determination of the properties of the isolate by the half-leaf test. The transmission of the virus isolate by seeds was not proved.

A study was made of the effect of different constant soil moisture (90, 60 and 30 or 40% maximum capillary capacity) on the net assimilation rate (N.A.R.) in maize in relation to changes in relative transpiration (R.T.), water saturation deficit of the leaves (W.S.D.) and osmotic pressure of the cell sap (O.P.). The soil moisture was maintained constant either from the planting of the germinating grain, or from the phase of 4-6 leaves. An attempt was made to interpret the mechanism of action of water deficit on photosynthesis and at a rough differentiation between the indirect effect through changes in internal diffusive resistance to carbon dioxide and the direct effect on the hydration of the photosynthetizing tissue. In plants exposed to different constant soil moisture levels from the phase of 4-6 leaves, the initial difference in N.A.R. corresponding to the degree of lowering of soil moisture gradually evened out during the vegetation season. On applying different constant soil moisture levels from the time of planting no marked differences in N.A.R. were found between plants cultivated at high values of soil moisture (60% and 90%). In plants cultivated from planting at 40% soil moisture, the course of changes in N.A.R. was qualitatively different from that of the above two variants and corresponded more or less to changes characteristic for the process of adaptation to unfavourable conditions. From the analogous course of N.A.R. and R.T. it can be assumed that in all cases the intensity of photosynthesis was very markedly influenced by changes in diffusive resistance to carbon dioxide. On the basis of an analysis of changes in O.P. of the cell sap and W.S.D. of the leaf tissue, the assumption was made that in plants cultivated from planting at 40% and to some extent at 60% soil moisture, irreversible adaptation changes occurred in the structural conditions of photosynthesis as a result of continuous dehydration. In plants cultivated at similar soil moisture levels from the phase of 4-6 leaves, the changes in the intensity of photosynthesis were more likely caused by actual dehydration of the photosynthetizing tissue.

The chlorophyll mutation rate on a large number of plants after the dose rates 0.004-16.800 R/day during the whole vegetation cycle was examined. The mutation frequency increases as early as after the dose rate 4 mR/day. The dose rate of approximately 8 mR/day is necessary to double the mutation frequency.

In studying the character of the relationship between the porphyrin metabolites containing iron and magnesium the catalytic activity of Fe-porphyrins and the possibility of their participation in chlorophyll formation in leaves and roots of plants was considered.
The results obtained elucidate the changes in the pigment content of leaves in the course of iron utilization by the plant (i.e. of the activity of iron-containing enzymes, synthesis of protohematin). The main trend of changes was observed in the following cases: (1) during utilization of various compounds affecting the individual components of the oxidation-reduction systems and the pigment content of the plant; (2) during changes of metabolism due to altered conditions of mineral nutrition: (3) in comparing the metabolism of variegated plants.
It is assumed that in all the cases the process of chlorophyll formation is directly associated with the processes of iron utilization by the plant both in the leaves and in the roots.
Data concerning the synthesis of protohematin in the isolated roots of some plants, on the activity of Fe-porphyrin enzymes therein and on the formation of chlorophylls during illumination of roots are presented. The question of the significance of synthesis of Fe- and Mg-containing porphyrins in roots for the metabolism of the whole plant is discussed.
The possible relationship between the processes of energy conservation or utilization by the cell and the equilibrium between the processes of pigment synthesis and catabolism in plastids are taken up.

The induced meiotic abnormalities as a result of sprayingVicia faba plants with aqueous saturated solutions of podophyllin and 8-hydroxyquinoline were studied. The 2 drugs induced the same types of abnormalities including lagging chromosomes, stickiness, and bridges. The main difference between the 2 agents was the induction of polyploid P.M.Cs. by 8-hydroxyquinoline.

A method suitable for serial determinations in genetic experiments is described. Colorimetric determination of HCN (by 1-phenyl-3-methyl-5-pyra-zolone and pyridine, phenolphthalin or sodium picrate) facilitates processing of samples releasing 0.2 μg to 20 μg of HCN. Before hydrolysis of the glycoside the oil content in the seeds can be determined. Variance analysis of the cyanogenic properties of the particular seeds of two sorts of flax with an average HCN content of 0.4 and 0.9 μg shows that the differences between plants and between capsulae of the same plant can be determined.

The lignine content and mechanical rigidity of siliquae ofBrassica napus L. ssp.oleifera f.biennis, cv. Třebíčská were studied. Experimental plants were harvested in five consecutive periods and developmental stages of siliquae were determined on the basis of the weight of 1000 seeds. During the development of siliquae a gradual accumulation of lignine takes place; the content reached its maximum in certain developmental phases and then decreased. The mechanical rigidity of siliquae had a similar tendency. There were statistically significant differences in both properties studied within individual developmental phases. A highly significant positive correlation was found between lignine content and mechanical consistency.

The effect of temperature on the number of lesions and the time of their appearance was studied by inoculating French bean leaves (Phaseolus vulgaris L. cv. Perlička) with lucerne mosaic virus either 24 or 48 h before or, 24 or 48 h after they were exposed to various temperatures. The temperatures tested were 23, 25, 27, 30, 33 and 36° C.
Before and after such exposures the plants were kept in a constant temperature of 25° C. By increasing the temperature before inoculation the number of lesions increased in comparison with the control. The optimal temperature for the maximum number of lesions is between 27° and 30° C. There is no significant difference between those experiments when the exposure time was 24 h or 48 h before inoculation. The same temperatures applied for 24 or 48 h after inoculation have a decreasing effect upon the number of lesions formed by LMV on French bean leaves. The decrease is 30 to 75%. In this case the first necrotic local lesions appeared 42 h after inoculation when exposed to higher temperatures above 27° C for 24 h, and 60 h after inoculation when exposed to these temperatures for 48 h. The shape of lesions varied a little in both cases as the pictures show.

Development of the shoot apex up to floral differentiation was investigated in the short-day plantChenopodium rubrum. The changes occurring in the apex from energence until full opening of the cotyledons (Figs 1-4), development during photoperiodic induction (Figs. 5-8), as well as the resulting floral differentiation (Figs. 9-10) are described. It was aimed at excluding the influence of plastochron changes on the interpretation of ontogeny of the shoot apex. For that reason two planes of longitudinal sections and two plastochron stages were compared.
In young plants zonation does not become fully evident prior to floral differentiation. The anatomical structure of the shoot apex does not change substantially during the first two inductive cycles which proved to be obligatory under the given experimental conditions. The changes occurring during two further inductive cycles correspond to the total activation of the meristems as manifested by the growth and branching of the apex preceeding floral differentiation proper.

The effect of 2-thiouracil on vegetative growth and floral differentiation was investigated inChenopodium rubrum plants grown in water cultures. Between the low concentrations of the agent, stimulating vegetative growth and floral differentiation, and those inhibiting both these processes, a narrow concentration range was found (1.10^-5 m to 2.10^-5 m), where growth was inhibited selectively. At a concentration of 1.10^-4 m a selective inhibition of development was found when 2-thiouracil was applied at the beginning of photoperiodic induction. Inhibition of development was strong regardless of whether 2-thiouracil was applied before, during or closely after 4 days of photoperiodic induction; the degree of growth inhibition, however, changed in dependence on photoperiodic induction. The strongest relative inhibition of development, calculated as a ratio between development and growth, was observed always at the beginning of photoperiodic induction.
Investigation of plant growth as well as the anatomical and autoradiographic study after the application of 2-thiouracil indicate that the inhibition becomes evident at the end of 4 days of application by an overall growth inhibition and a decrease of mitotic activity. Reversal by uracil was possible after simultaneous application of 2-thiouracil. The nature of the selective inhibition is discussed and two possible interpretations of the data obtained are analyzed: a) different response of growth processes in apices and young vegetative organs respectively with regard to different participation of cell division and elongation, b) specific inhibition of floral differentiation.

The application of CCC at concentrations inhibiting flowering ofChenopodium rubrum reduces the level of endogenous gibberellins in the apical buds of the plants. The effect of CCC may be reversed by appropriate concentrations of gibberellin (GA-), indole acetic acid (IAA) or kinetin. Kinetin applied to the apical bud during floral induction reduced the level of endogenous gibberellins similarly as CCC and if both CCC and kinetin were applied simultaneously their action was additive. On the other hand IAA applied under the same conditions increased the level of endogenous gibberellins and after joint application of CCC and IAA their level was the same as in untreated control plants. After application of CCC during floral induction the level of endogenous auxins did not change markedly but an active substance "x" appeared on the chromatograms of indole compounds. This substance was found also after simultaneous application of GA- and CCC but not after joint application of CCC and kinetin. If follows from our results that the same morphological phenomenon (flowering) can take place in plants considerably differing as to their level of endogenous growth substances. The ratio of different growth substances is obviously more important than the actual level of the single substances.