PHOTOPERIODIC EFFECTS IN WOODY PLANTS 227 



tation has been performed ), the development of the axillary buds is re- 

 duced, the most inhibited bud being the one situated in the axil of the 

 shaded leaf (Fig. IB). If both the two top leaves are given short days, 

 then the growth of the two axillary buds is more strongly inhibited, al- 

 though ah the rest of the plant continues to receive long days (Fig. 

 lC)."when the whole plant is kept under short days before and after 

 the operation, decapitation causes no growth whatsoever in the axillary 

 buds (Fig. ID). Thus, this first set of experiments shows that the top 

 young leaves, having reached approximately their full size, are able 

 to inhibit strongly the development of buds in their axils when they, 

 alone, are subjected to short days. 



Prevention of Growth Inhibition through Leaf Removal 



Let us now consider another photoperiodically sensitive plant, 

 Weigela florida, clone Eva Rathke. This time, we will use plants which 

 have two equal branches and which have been growing vigorously 

 under long days. We will divide them into five equal groups. At time 

 0, we place groups A-E under short, 9-hr days, after having treated 

 them in the various ways depicted in Fig. 2. For about 2 weeks, there 

 is some residual growth which we will not consider here, but, after 

 that time, the effect of the short-day treatment becomes fully visible. 

 We will measure, therefore, the new growth some 23, 37, and 50 days 

 after the beginning of the short-day treatment. The plants of group A, 

 which have all their leaves on, stop growing completely; no stem 

 elongation and no development of new leaves occur, at least from 23 

 days on after the beginning of the short-day treatment. On the con- 

 trary, the plants of group B from which all the leaves have been 

 stripped (the newly unfolded leaves being removed as they reach about 

 three-fourths of their full size), continue to grow, and produce new 

 stem and new leaves. Plants of group C, in which only the three first 

 pairs of leaves are kept removed, also continue to grow under short 

 days, although at a slower rate than the plants of group B. However, 

 the plants of group D, which had all their leaves removed except the 

 first pair (not counting leaves less than 1 cm long) stop growing com- 

 pletely under short days, as did those of group A which had all their 

 leaves on. The above-mentioned results, which confirm those of Downs 

 and Borthwick (1956), allow the two following conclusions to be 



