THE ANGIOSPERMAE : STEMS 



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made to relate them to known evolutionary facts. We have sufficiently 

 expressed our views on this aspect of the subject in Chapter I. 



In the great majority of plants the leaf-bearing axis and the leaves 

 collectively constitute the shoot. In the preceding Chapter on Roots we 

 referred to certain cases of rootless plants. Analogous cases occur of plants 

 in which the axis is lost. For example, the seedling of Streptocarpus begins 

 with the normal dicotyledonous form. One cotyledon and the stem apex 

 rapidly abort, and in some species the hypocotyl also, while nothing more 

 develops except an adventitious inflorescence from the base of the single 

 cotyledon, which itself becomes the sole foliage leaf. 



The case of the Duckweed {Lemna) is still more extreme (Fig. 830). The 

 entire plant consists of minute floating segments which proliferate by budding 



Fig. 830. — Lemna tristdca. Floating plant showing proliferation by budding 

 from the segments. {After Goebel.) 



from their edges. Whether these segments should be classed as leaves or 

 stems has been much discussed, although, as we have seen above, there is 

 reason to believe that the distinction is one of name only. Comparison with 

 the nearest related genus Pistia, however, confirms the view that they are 

 equivalent to leaves and thus that the plant has no axis. 



The principal function of a normal stem is unquestionably that of dis- 

 playing the leaves, and there are scarcely any stems, however much altered 

 from the normal form, which do not bear leaves of some sort, although they 

 may be reduced in certain cases. The possession of leaves or leaf scales may 

 indeed be regarded as the most definite distinguishing character of a stem. 



In addition to supporting leaves the stem must also provide channels of 

 supply for the movement of fluids both to and from the leaves, and the internal 

 anatomy of the stem is suited to this function. Stems also serve a number of 

 subsidiary functions, though not in all cases. These are, briefly: (i) Storage, 

 either of food materials, or of water, or both. (2) Associated with this is the 

 function of perennation, that is, of tiding the plant over the season of winter 



