XEKOPHYTE SOCIETIES 
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 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 
132), or "peat-bog," is included 
among hydrophyte societies. It has 
an abundance of v/ater, 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 
than do the societies 
which are grouped ac- 
cording to the water 
supply. 
Societies 
I 
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. 
No attempt will be 
made to classify these 
very numerous socie- 
ties, but a few prom- 
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). 
