ECOLOGY 8 1 



water, firmly rooted in the mud, but are erect, self-supporting 

 and rise tall and slender, high above the surface of the water. 

 The special adaptation here is in the height of the plant, which 

 permits considerable change in level of the water surface with- 

 out drowning the plant. 



192. Marsh and swamp plants do not differ much from the 

 mesophytes in structure, but nevertheless the continually 

 saturated soil and other conditions which obtain in such locali- 

 ties, are sufficiently different from mesophyte conditions, on 

 the one hand, and true hydrophyte conditions on the other, to 

 give the floras of swamps and marshes a character of their own. 



193. Comparing xerophytic plants with hydrophytes we find 

 that in a number of particulars they present the opposite ex- 

 tremes of structures. Thus the epidermis of xerophytes is 

 extremely well developed, often consisting of several layers of 

 cells and provided with a very thick cuticular wall on the sur- 

 face. Stomata are less numerous. The plant as a whole is 

 more compact, thus reducing the ratio of surface to volume. 

 All these peculiarities result in decreased loss of water by 

 transpiration and evaporation and are clearly an adaptation to 

 scanty water supply. The massive form of these plants also 

 affords space for the storage of water obtained from occasional 

 showers. 



194. Under semi-arid conditions there is sometimes another 

 device employed for preventing the excessive loss of water, 

 namely, the vertical or meridional position assumed by the 

 leaves or leaf-like organs (phyllodes) and the consequent tem- 

 pering of the force of the sun's rays. 



195. In those regions of the earth's surface which have 

 alternately wet and dry seasons the vegetation also presents 

 alternately mesophytic and xerophytic characters. This docs 

 not mean merely that at one season mesophytes are prominent 

 and at another the xerophytes, but even the same plant alters 



in character with the seasons. 

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