CHAPTER VIII. 



THE SHOOT. 



98. The gametophyte shoot. — If plants could be examined 

 in the order of their development, it would be discovered 

 that the shoot has been evolved earlier than the root. It 

 makes its appearance first in the leafy liverworts and in the 

 mosses, in which the gametophyte and sporophyte each form 

 a stem. The gametophyte differentiates its secondary shoot 

 into a stem and leaves. This stem in liverworts is a slender 

 cylindrical body of very simple structure, upon whose flanks 

 arise leaves which consist of a single layer of cells only. 

 (See ^j 60.) Neither the stem nor the leaves are homologous 

 with the stem and leaves of the higher plants. In the stem 

 itself one finds all the cells practically alike, so that little 

 differentiation of tissues has yet occurred. In mosses, how- 

 ever, the gametophyte stem shows some advance, in that its 

 tissues are clearly differentiated, the outer being transformed 

 into thick-walled cells, in order to give mechanical rigidity 

 to the stem, while the innermost, remaining slender, are 

 much elongated and serve the purpose, it may be, of con- 

 duction. (See ^f 63.) This differentiation is naturally more 

 marked in those mosses which are erect and whose body 

 becomes largest, since in these the need for rigidity and con- 

 duction of food materials from one part to the other becomes 

 greater. In both groups the branching of the gametophyte 

 shoot is like that of the sporophyte shoot of some of the 

 higher plants, except that the branches never stand in the 

 same relation to the leaves. (See ^| 65.) 



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