. The Characeae of America. 24 ,>' I j<rv 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
. The Characeae of America. 24 ,>' I j<rv 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 Fig. Pl
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