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CHAPTER XIII 



THE MORPHOLOGY AND VASCULAR ANATOMY 



OF AQUATIC STEMSi 



THE stems of plants that pass the greater part of their 

 vegetative life entirely submerged, fall in general into 

 two categories. The less common type is the abbreviated axis 

 bearing a tuft of long narrow leaves (e.g. Stratiotes, Fig. 31, 

 p. 49 and Fig. 32, p. ^2) while, on the other hand, the 

 majority of submerged plants are characterised by thin, elon- 

 gated, branched stems rising wholly or partially into the water, 

 clothed with leaves and often capable of rooting at the nodes 

 (e.g. Potamogeton, Fig. 37, p. 60 and Myriophyllum, Fig. 144, 

 p. 221). Owing to the high specific gravity of the water, and 

 the lightness of the stems, due to the air in the intercellular 

 spaces, each axis is to a large extent relieved of the task of 

 supporting the weight of its branches. In consequence there 

 seems to be no impulse to the relatively strong development of 

 a single main axis, and, in conformity with this, the general 

 system is often sympodial (e.g. Hippuris, Fig. 112). The plant 

 frequently grows actively in front while it dies away behind, 

 and may thus be regarded, to use Schenck's expression, as being 

 in a state of perpetual youth. The older regions tend to become 

 infested with a flora of epiphytic Algae and Fungi, among which 

 a microscopic fauna makes its appearance. This is an obvious 

 disadvantage, since no leaf thus laden can perform its functions 

 successfully. Possibly the rapid growth of fresh leafy shoots 

 at the apex serves as a compensation for a loss of activity in the 

 older regions, traceable to this cause. 



The vascular system of submerged stems shows certain 

 modifications upon the terrestrial type, the most striking differ- 



1 For a detailed treatment of this subject see Schenck, H. (1886), 

 which has been largely drawn upon in the present chapter. 



