208 



PLANT RELATION S. 



stands in the water its stem is exposed 

 to a heat that is often intense. 



The ordinary prairie (see §169) 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 

 §140), 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 structure. 

 The cause for this has not yet been 

 determined, although several sugges- 

 tions 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 



that show xerophytic 



structures belong to- ~s£~PC~J~ ~~CTT~T~ J i- — v' 



gether more naturally 



than do the associa- 

 tions that are grouped 



according to the water 



supply. 



Associations. 



Xo attempt will be Fig. 183. A section through a Begonia leaf, show- 



■, , -I •» ,i ing the epidermis (ep) above and below, the 



made tO ClaSSliy tnese wa ter-storage tissue (tea) above and below, and 



Very numerous aSSOCia- the central chlorophyll region (as). 



Fig. 182. Cells from the leaf 

 of a quill wort (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. 



