XEIIdl'in'TE SOCIETIES 



•2m 



stands ill tlie water its stem is exposed 

 to a lieat which is often intense. 



The ordinary prairie (see §146) is 

 inelnded among mesopli3'te societies 

 on account of tlie rich, well-watered 

 soil; and vet maiiv of the plants are 

 very xerophytic in structure, proljahlv 

 on account of the prevailing dry winds. 

 Tlie ordinary s[)]iagiuim-bog (see 

 §132), or •■ peat-ljog,'' is included 

 among liydrophyte societies. It lias 

 an abundance of water, and is not ex- 

 posed to blazing lieat, as in the case 

 of the bulrushes, nr to drying wind, 

 as in the case of ])rairie plants ; and 

 yet its plants show a xerophytic struc- 

 ture. This is found to be due, proba- 

 Idy, to a hick of certain iiii}iortant 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 

 which are grouped ac- 

 cording to the water 

 supply. 



Societies 



No attempt will be 

 made to classify these 

 verv numerous socie- 

 ties, but a few prom- 



Fi(i. 18^. Cells from the leaf 

 of a quillwort (Isoefes). 

 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. 



Flii. 18-3. -Vseclion through a Htrjoida leaf, show- 

 ing the epidermis (^y/) above and below, the 

 ^\-ater-storage tissue t^'i-') above and belo^^. and 

 the central chlorophyll region (a*). 



