12O 



BOTANY OF THE LIVING PLANT 



I 



gradually diminishes on the one hand towards the tip, and on the other 

 downwards, until a point is reached where it ceases altogether. This 

 observation at once accounts for the general contour shown by growing 

 shoots. In the apical bud the leaves are closely grouped because the axis 

 has not yet extended to its full dimensions. The length of the inter- 

 nodes (or intervals between the leaves) increases downwards till their 



growth is complete, and their full length 

 has been attained. Below that point 

 the stem has become rigid, owing to 

 general thickening of the cell-walls as 

 they mature ; and this naturally makes 

 further elongation impossible. Thus 

 the longitudinal extension of the tissues 

 of a growing stem is a transitory state 

 between the period of initiation in the 

 embryonic region of the bud and the 

 mature and rigid state seen in the lower 

 part of the stem. Similar extension 

 may be seen in leaves and in roots. In 

 the latter it is found that the region 

 of most active growth is within a few 

 millimetres of the extreme tip. The 

 zone of extension is much shorter than 

 that usual in the stem (Fig. 84). 



Such extension of the whole part 

 depends upon the growth of the individ- 

 ual cells composing it. It has been seen 

 already (p. 21, Fig. 12) how young cells 

 may elongate greatly as they mature, 

 with vacuolisation of their cytoplasm. 

 During the process the turgor of the 

 cell stretches the thin cell-wall, which, 

 being plastic, yields. This would tend 

 to diminish the thickness of the wall, like a sheet of stretched rubber, 

 were it not for the fact that fresh cell-wall-substance derived from the 

 cytoplasm is continually being deposited. In point of fact the thick- 

 ness of the cell-wall in any maturing tissue may be seen actually to 

 increase as the extension proceeds. Its increase in substance, chiefly 

 or perhaps wholly by apposition of fresh material to its surface, is 

 actually more rapid than its thinning off by stretching. The walls 

 are thickened by using up available materials, such as starch and sugar, 



FIG. 84. 



Localisation of growth near to the 

 root-tip of Vicia Faba. In I. the root- 

 tip has been marked with 10 zones i mm. 

 apart. In II. the same root after 

 22 hours. The lines nearest the tip are 

 now most separated owing to the growth 

 having been there most active. (After 

 Sachs, from Strasburger.) 



