318 FOUNDATIONS OF BOTANY 
from Part I of this book about the forms, structure, and 
habits of ordinary plants, together with what the student’s 
own observation, aside from the study of botany, has taught 
him, should suffice to give him a fair idea of ae 
plant life. 
The typical mesophyte of the northern United States is 
an annual, since most of our larger perennials pass the 
winter in a xerophytic condition, to avoid destruction by 
drying up during the long period when 
the roots can absorb little or no water 
FiG. 225.— Cross-Section of Leaves of a Grass,' unrolled for Exposure to 
Sunlight and rolled up to prevent Evaporation. 
r, ridges of the upper epidermis, with many stomata on their surfaces ; 
e, thick lower epidermis, without stomata. 
from the frozen soil. Our evergreen coniferous trees, 
such as pines, spruces, cedars, and so on, have leaves of 
decidedly xerophytic structure. So also do such ever- 
green shrubs as the rhododendrons, wintergreen, arbutus, 
holly, and bearberry. Our deciduous trees and shrubs and 
most perennial herbs are tropophytes (Sect. 390). 
390. Tropophytes, or Seasonal Plants. — Examples of 
these are most deciduous trees and the majority of the 
perennials of temperate regions, for instance oaks, elms, 
birches among trees, and tulips, crown imperials, lilies, 
hyacinths, spring-beauties, peonies, dahlias, and potatoes 
among herbs. Such plants have a pretty large surface for 
1 Stipa capillata. 
