226 



A. M. Kuziisr 



m 



I 1 Control, completely 



I 1 screened plant 



V~~\ Experiment, screening 



of the points of growth, 

 single leaf irradiated with 

 25 kr 



Apical Meristem 

 point of ceils of 

 growth the root 



Fig. 2. — The decrease in the mitotic index one or two days after irradiation of a single 

 leaf of Vkia fah(t.. The data of (i experiments have been averaged. 



eliminated any possible slight effect of scattered radiation in the 

 chamber. 



A distant decrease of mitotic index in the points of growth lead to a 

 60 per cent inhibition of growth and development of the whole plant as 

 compared with the control plants. 



It was snggested that the radiation disturbed the balance of metabolic 

 processes in the leaf, so that the concentration of certain metabolites, 

 probably present in small quantities normally, greatly increased. We 

 thought that these metabolites normally acted only inside a leaf cell, 

 inhibiting cell division, but that, after irradiation the excess rushed 

 together with the other assimilates to points of growth and there 

 demonstrated their antimitotic activity. In order to check this sugges- 

 tion, the leaf was immediately removed after local irradiation. A corre- 

 spondmg non-irradiated leaf was removed from the control plant. No 

 inhibition of mitotic activity at the points of growth was observed. 

 When the exposed leaf was removed 4 hr after irradiation slight inhi- 

 bition of mitosis in the apical point of growth was observed, but none in 

 the roots. The removal of the leaves after 24 hr already had no effect 

 and the plants showed great inhibition of mitosis after 2 days both in 

 the apical points of growth and in root meristem, as shown in Table II. 



We attempted to extract the active substances from the leaves. 

 Normal and irradiated leaves (20,000 r) were frozen in liquid nitrogen, 

 pounded together with pure quartz sand and extracted with hve 

 volumes of water for 24 hr at 0°C'. The extracts were filtered and roots 

 of four-day-old normal sprouts of Vicia faba were immersed in this 

 medium for 24 hr. 



For control, other roots were immersed in water from the tap. After 

 20 hr the roots were fixed and the mitotic index determined. The 

 results are given in Table III. The data show that extracts from 



