2 4 



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productive organs and of cortical development. In Fig. 18 young 

 growing leaves are represented with commencing differentiation of 

 cells, "n'," nodal cells ; " in," internodal cells ; "a'," the apical cell, 

 which, with the two cells immediately beneath, form the tip of the 

 leaf. The alternation of nodes and internodes in the leaf is restricted, 

 and the number of nodes is very constant for each species ; in this 

 respect the growth of the leaf is quite different from that of the stem. 

 In Nitellse the number 

 of divisions of the leaf 

 rarely exceeds five; in 

 Chara it may be as 

 many as fifteen. (Ni- 

 tella flexilis, two articu- 

 lations ; N. mucronata, 

 three ; N. megacarpa, 

 four or even five ; in 

 Tolypella, often six ; in 

 Chara gymnopus Mi- , 

 chauxii, twelve to fif- 

 teen.) 



The internodal cells 

 of the leaf, like those of 

 the stem, increase in 

 length, but do not sub- 

 divide. 



The nodal cell divides, 

 by excentric fissation, 

 into a circle of marginal 

 cells, which enclose a central cell; the formation of these marginal 

 cells takes place, alternately, on either side of a point on the ventral 

 (toward the axis of the plant) aspect of the leaf, until the circle of cells 

 closes on the dorsal side. 



The ventral cell is, therefore, the oldest, and from it the fruiting 

 organs are always developed. These nodal cells arise exactly in a 

 line, and do not deviate, as do those of the stem, hence there is no 

 torsion of the leaf, as of the stem (or, if, as rarely happens, a slight 

 twisting is observed, it is in the opposite direction to that of the stem, 

 and is due to the habit of growth, and not to any morphological pe- 

 culiarity). From the nodes of the leaf lateral rays or bracts arise ; in 

 Nitellif the lateral rays usually equal the main leaf in size and look 

 like the leaf. This is well seen in our N. flexilis, in which the lateral 

 ray so simulates the normal leaf, that the leaf appears simply forked. 



Fig. 18. 



