166 BOTANY PART i 



absorb both water and nutrient salts and also the necessary gases 

 (carbon dioxide and oxygen) from the water by the whole surface 

 of their stems and leaves. In considering the conditions of life in 

 water it is essential to know the amounts of various gases which can be 

 dissolved and to contrast this with their presence in the atmosphere. 

 One litre of air contains about 210 c.cm. oxygen and 0'3 c.cm. of carbon 

 dioxide. In one litre of water at 20 C., on the other hand, there 

 can be dissolved only about 6 c.cm. oxygen, but 0'3 c.cm. carbon 

 dioxide. There is thus available for the submerged plant as much 

 carbon dioxide, or even somewhat more. There is, however, little 

 oxygen, especially in the case of still water, since the diffusion of 

 this gas in water is very slow. 



Roots may be absent (Utricularia, Ceratophyllum, Wolffia) or 

 only serve to attach the plant to the soil. The shoot, on the other 

 hand, has become similar to a root, in that the thin walls of its 



epidermal cells have a 

 very thin cuticle that 

 offers little hindrance 

 to the entrance of water. 

 The large surface ex- 

 posed by the fine sub- 

 division of the lamina 

 of the submerged leaves 

 (fiatrachium, Fig. 138, 



PIG. 187. Transverse section of the leaf of Zannichellia palustris. Utricularia, MyriOpliyl- 



(x 146. After SCHENCK.) i um ^ Ceratophyllum) 



stands in relation to 



the slowness of the diffusion of gases in water ; floating and 

 aerial leaves of water plants, on the other hand, are typically 

 formed (heterophylly, cf. p. 11 6). As regards their anatomy 

 the submerged leaves are characterised by the absence of stomata, 

 and usually of hairs from the epidermis, the cells of which 

 contain chlorophyll; the mesophyll has large intercellular spaces, 

 and consists of uniform parenchyma, .not showing the distinction 

 of palisade and spongy tissue. The leaves in transverse section 

 thus appear bilaterally symmetrical (Fig. 187). The feeble develop- 

 ment of water -conducting elements in the stems and leaves, and 

 the absence of secondary thickening, are related to the absence of 

 transpiration, and of active transport of water. The support afforded 

 by the surrounding water renders mechanical tissues unnecessary ; 

 the pulling forces exerted in quickly -flowing water are met by the 

 central position of the vascular bundle. 



The great development of the intercellular spaces is a striking 

 feature of almost all aquatic and marsh plants. They are wide, and 

 form a regular system of air-filled chambers and passages, which are 

 separated by parenchymatous partitions, usually only one cell thick : 



