68 



GENERAL BOTANY 



-teams 



-Bud scales - 



pairs of leaves at the nodes are very short, as shown in the long 

 section of the bud b. The meristem terminates the axis of the 

 bud, all of which will constitute the elongating zone when 

 growth first starts in the spring. The expansion of the inter- 

 nodes by growth separates the leaves (c and d), which at first 

 grow more rapidly on their inner than on their outer surfaces, 



and so unfold and spread 

 out horizontally to the light. 

 As growth proceeds the cells 

 within the lower internodes 

 reach their definite length and 

 thus add new segments to the 

 permanent portions of the 

 young stem, as in the case 

 of roots. In this permanent 

 portion the cells differentiate 

 and the main tissues of the 

 mature stem are formed. Be- 

 fore growth in length ceases, 

 the buds for the next season 

 become apparent and proceed 

 to form the young leaves for 

 the growth of the following 

 spring. Indeed, the growing 

 points, or meristems, for these 

 buds are apparent, in the 

 lilac, in the wintering bud in 

 the axils of its rudimentary leaves. In the autumn, therefore, 

 when the leaves fall, the lilac shoot of the season presents the 

 aspect shown in Fig. 38, 5, with all the buds laid down and 

 protected by bud scales. 



In all these changes incident to growth the cells and organs 

 of plants are subject to the effect of environmental forces. 

 A certain amount of water and heat, varying with the type 

 of organism, is necessary for growth. Light usually has a 

 retarding effect on growth, although this too is a variable 

 factor in its influence. In general it may be said that extremes 



Leaves , 



ud scales 





FIG. 37. Bud structure and growth in 

 the lilac 



a, 6, surface and sectional views of a bud 



in the resting condition ; c, d, similar views 



of the same bud in spring when it first 



begins to grow 



