S12 FOUNDATIONS OF BOTANY 
is in dividing mesophytes from the other two classes, into 
which they shade by indefinite gradations. In order to 
know whether the plants of a region have plenty of water 
or not, we must know not only how many inches of yearly — 
rainfall there are, but also what the soil is like, what is 
the temperature of the soil and air, whether or not there 
are dry winds, and whether there are 
fogs or heavy dews. A lichen on a 
bare rock may be living almost under 
Fic. 218. — Aquatic Plants: Pond-Lilies with Floating Leaves and 
Sedges with Aérial Leaves. 
desert conditions, while a pitcher-plant in a bog near by 
has its roots in standing water (or in ice) nearly all the 
year round. 
384. Hydrophytes. — Some of these are herbaceous 
aquatic plants, like the duckweed, the pickerel-weed, 
the pond-lily, and the water-crowfoot ; others, such as the 
“calla” (Richardia), the buckbean, the cat-tail, and the 
sweet flag, many ferns, mosses, and liverworts, prefer 
