208 



PLANT RELATIONS. 



stands in the water its stem is exposed 

 to a heat which is often intense. 



The ordinary prairie (see §169) is 

 included among mesophyte societies 

 on account of the rich, well-watered 

 soil; and yet many of the plants are 

 very xerophytic in structure, probably 

 on account of the prevailing dry winds. 



The ordinary sphagnum-bog (see 

 §140), or " peat-bog," is included 

 among hydrophyte societies. It has 

 an abundance of water, and is not ex- 

 posed 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. This is found to be due, proba- 

 bly, to a lack of certain important soil 

 materials. 



It is evident, therefore, that xero- 

 phytic structures are not necessarily 

 confined to xerophytic situations. It 

 is probably true that all societies which 

 show xerophytic structures belong to- 

 gether more naturally 



_^ 



Fig. 182. Cells from the leaf 

 of a quilhvort (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. 



than do the societies 

 which are grouped ac- 

 cording to the water 

 supply. 



Societies. 



No attempt will be 

 made to classify these 

 very numerous socie- 

 ties, but a few prom- 



COS 



Fig. 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). 



