XEKOPHYTE ASSOCIATIONS 



203 



stands in the water its stem is exposed 

 to a heat which is often intense. 



The ordinary prairie (see §146) is 

 included among mesophyte associa- 

 tions on account of the rich, well- 

 watered soil ; and yet many of the 

 plants are very xerophytic in struc- 

 ture, probably on account of the pre- 

 vailing dry winds. 



The ordinary sphagnum-bog (see 

 §132), or " peat-bog," is included 

 among hydrophyte associations, It 

 has an abundance of water, and is not 

 exposed to blazing heat, as in the case 

 of the bulrushes, or to drying wind, 

 as in the case of prairie plants ; and 

 yet its plants show a xerophytic struc- 

 ture. The cause for this has not yet 

 been determined, although several 

 suggestions have been made. 



It is evident, therefore, that xero- 

 phytic structures are not necessarily 

 confined to xerophytic situations. It 

 is probably true that all associations 

 which show xerophytic 

 structures belong to- 

 gether more naturally 

 than do the associa- 

 tions which are 

 grouped according to 

 the water supply. 



Fig. 182. Cells from the leaf 

 of a quillwort (Isoetes). 

 The light is striking the 

 cells from the direction of 

 one looking at the illus- 

 tration. If it be some- 

 what diffuse the chloro- 

 plasts distribute them- 

 selves through the shal- 

 low cell, as in the cell to 

 the left. If the light be 

 intense, the chloroplasts 

 move to the wall and as- 

 sume positions less ex- 

 posed, as in the cell to 

 the right. 



Associations 



No attempt will be 

 made to classify these 

 very numerous associa- 



Fiq. 183. A section through a Begonia leaf , show- 

 ing the epidermis (ep) above and below, the 

 water-storage tissue (ws) above and below, and 

 the central chlorophyll region (as). 



