98 



General Botany 



when the leaves have developed to a certain rather definite size. 



The leaf starts as a small protuberance on the side of the apex 



of the stem. The mass of cells 

 that make up this protuberance 

 are all similar. As growth pro- 

 ceeds, cell division, cell enlarge- 

 ment, and cell differentiation take 

 place and the five tissues of the 

 leaf are formed. At first, then, 

 all parts of the leaf are growing. 

 After the leaf is mature, further 

 enlargement will not take place, 

 no matter how favorable to 

 growth the external conditions 

 may be. The question arises, do 

 all parts of the leaf mature at 

 the same time, or does growth 

 continue in some parts longer 

 than in others? There is one 



sunflower (C). The fern leaf grows at the characteristic of grOwing tisSUe 



apex; the leaf of the grass, as is common . •^^ ^ i 



in paraUel-veined leaves, grows at the that Will help US m anSWCHUg 



base ; the sunflower leaf, like other net- this inquiry : yOUng tisSUC is VCry 



veined leaves, grows in all its parts. . -, i -i i_ i um 



tender and easily broken, while 

 old tissue is stronger and firmer (Figs. 51 and 72). 



Fern leaves grow at the apex. The fern leaf is one that 

 may be studied in this connection, for the growing portion 

 is not only tender but coiled up, and its unfolding may be 

 noted from day to day by marking with India ink the suc- 

 cessive positions of the coil. In the Boston fern, which is 

 so commonly cultivated as a window plant, the leaf may con- 

 tinue to unfold for weeks, if the water supply is adequate and 

 other conditions are favorable. Evidently in the ferns the last 

 growing region is at the apex and the older part of the leaf is 

 the base. If the tip of a fern leaf is injured, further growth 

 is stopped. 



Fig. 51. Growing regions (shaded por- 

 tions) of leaf of fern {A), grass (B), and 



