562 GROWTH, ASSIMILATION, AND ACCUMULATION 



A 



A 



A/V\ 



W 



A 



V 



V 



V 



D C 



Fig. 121. Diagram illustrating two methods by which cambium cells divide result- 

 ing in the formation of additional cambium cells. {.-J) and (B) two stages in the 

 radial division of a cambium cell. (D) to (G) four stages in the "pseudo-transverse" 

 division of a cambium cell followed by a sliding of the two derivative cambium cells 

 past each other. 



Development of Lateral Organs. — Practically all of the lateral organs 

 borne on stem and root axes develop during primary growth. Leaves origi- 

 nate from the leaf primordia which are small protuberances which develop 

 laterally at more or less regular intervals from the apical meristem (Fig. 117). 

 The point on a stem at which a leaf or leaves develops is called a node; 

 the intervening stem segments between the nodes are called internodes. The 

 histogenic development of a leaf from its primordium does not follow the 

 same course in all species (Foster, 1936) although there are many points 

 of similarity in the development of most kinds of leaves. As an example 

 we may consider the development of the tobacco leaf (Avery, 1933). The 

 apical leaf meristem in this species consists of a single cell which continues 

 to produce new cells until the leaf is 2-3 mm. long. Until the leaf is about 

 0.6 mm. long it consists of only a midrib primordium. The lamina originates 

 from two rows of subepidermal meristematic cells, one on each side of the 



