454 



PLANT PHYSIOLOGY 



cereals (Fig. 139). A detailed study of the growth of wheat, 

 for example, shows that during the first weeks after germination 

 of the seeds, the stems remain short, forming the so-called 

 ''tillering node," w^hen the leaves alone develop. But when 

 the internodes and the inflorescence have completed their 

 embryonic development, then a rapid elongation of the inter- 

 nodes, or shooting, begins. The inflorescence is pushed out of the 

 tubelike leaf sheaths, which until then have tightly enclosed it. 

 In biennials, these two growth stages are separated by the 



winter period, when the plant 



has entered into a state of rest. 



The transition of trees into a 



h't^ state of rest, which generally 



\--'^ occurs in autumn, is usually 



•{ accompanied by a shedding of 



t 



Fig. 139. — Terminal bud of a young 

 maize plant. 



Fig. 140. — Abscission layer formed 

 across a petiole at t on each side of 

 vascular bundle n. 



leaves and sometimes of whole twigs, mostly the shorter ones. 

 This shedding represents a normal physiological process and is 

 caused by the formation of a special secondary meristem, the 

 so-called ''abscission layer" (Fig. 140) in the leaf peduncle. 

 The cells of this layer become rounded and separate from each 

 other, the leaf remaining attached only by means of the bundle 

 of conductive elements. Finally, they also rupture, and the leaf 

 is shed. 



The causes of the formation of an abscission layer in the period 

 of leaf fall are not clear as yet. At any rate, it is not the direct 



