422 



LECTURE XXVI. 



The leaves which have been developed at the growing-point of a shoot, generally 

 grow much more rapidly than the parts of the latter situated above their insertion on 

 the shoot-axis. In this lies the cause of the formation of buds at the end of the shoot 

 (Fig. 258). 



In the leaves also various changes in form are brought about during the process 

 of elongation. The elongation in the leaves may either begin at the apex and end 



at the base, or conversely; and in compli- 

 cated forms of leaves much more complex 

 distributions of growth may occur. 



This phase of growth by elongation 

 effects also the most various changes of 

 form during the increase in volume, espe- 

 cially in the case of shoots. The entire 

 form of a mature shoot depends in general 

 essentially upon the processes of con- 

 figuration which take place during elon- 

 gation. Contrasted with the growing-point, 

 however, this phase of growth is distin- 

 guished by the fact that no more new 

 organs are developed : only those already 

 present obtain their definitive size and 

 form. 



While the extremely slow growth of 

 the embryonic tissue takes place by means 

 of a proportionate addition of proteid sub- 

 stances and nuclein ; elongation, on the 

 contrary, consists in an enlargement of 

 the cells which is chiefly effected by the 

 addition of water. The cells which in the 

 embryonic condition are solid, and consist 

 merely of protoplasm and nucleus, change 

 during elongation into vesicles filled with 

 water, the volume of which may attain a hun- 

 dred or a thousandfold that of the original 

 embryonic cells : in this, however, no pro- 

 portional increase of the organic substance 

 takes place, but, in the main, the intercalation of water only. This, however, 

 does not preclude that the still thin cell-walls also have cellulose deposited in them, 

 and that the protoplasm may increase slightly in quantity. The enormous inter- 

 calation of water may, however, go so far that shoot-axes, leaves and root-fibres just 

 fully grown contain more than 90 per cent., or even more than 95 per cent, of their 

 weight and volume of water. 



During this active increase in volume of the cells, especially at first, numerous 

 additional cell-divisions take place : subsequently, however, these become rarer, and 

 gradually cease with the cessation of elongation. 



The elongation commences with the differentiation of the primitively homo- 



Fig. ■2-:ß.—F,quisetuin arvensc. Longitudinal 

 subterranean bud in March, s s apical cell of sten 

 leaves ; K K' two lateral buds exposed by the sectior 

 subsequently very long intemodes of the shoot-axis are 

 formed (slightly magnified). 



b—gb the 



ion. The 



yet 



