208 



PLANT RELATIONS. 



stands in the water its stem is exposed 

 to a lieat which is often intense. 



The ordinary prairie (see §163) is 

 included among mesopliyte societies 

 on account of the ricli, well-watered 

 soil; and yet many of the plants are 

 very xerojohytic in structure, probably 

 on account of the prevailing dry winds. 



The ordinary sphagnum-bog (see 

 §130), 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 ]u-airie plants ; and 

 yet its plants show a xerophytic struc- 

 ture. This is found to be due, jDroba- 

 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 



than do the societies •f'S"''\_/~~~Cn T ITT! — 4^ 

 which are grouped ac- 

 cording to the water 

 supply. 



Societies. 



Fig. ivr. Cells from the leaf 

 of a quillwort (Jsoetes). 

 Tlie 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. 



No attempt will be 

 made to classify these 

 very numerous socie- 

 ties, but a few prom- 



— U^ 



Fla. 178, A section tlirough a /Ur,oiii,i leaf, show- 

 ing the epidermis {ep} above and below, tlie 

 water-storage tissue iws) above and below, and 

 the central chlorophyll region (ns). 



