28 INTRODUCTION TO BOTANY 
Sandy soils contain more abundant air spaces than compact 
clays, and in clayey regions one important reason for deep and 
thorough cultivation of the soil is to insure the free access of 
Fic. 19. 
Effect of 
water supply on growth 
deficient 
The plant is groundsel, acommon 
European weed which grows to 
from 12 to 18 inches high. At D 
is shown the relative height of 
the same plant when grown in 
very dry sands along Mediter- 
ranean beaches. Modified after 
“Flora Danica” 
air to the roots of crops. 
26. Water supply of earth roots. 
It is known to most people that or- 
dinary plants must absorb through 
their roots a good deal of water. If 
house plants are left unwatered for 
two or three days, they begin to 
wilt. Field crops and sometimes 
even shade trees do the same in 
times of severe drought. Many 
plants, when grown in dry ground, 
will flower and seed in a dwarfed’ 
condition. The common groundsel, 
a weed not infrequently found in 
long-tilled fields and about door- 
yards (fig. 19), under favorable 
conditions grows to be a foot or 
more in height, but in the very dry 
sand along Mediterranean beaches 
this plant flowers and seeds when 
only an inch high. 
27. Roots of desert plants. The 
plants of desert and other dry 
regions frequently show striking 
peculiarities of form and structure, 
and among these is an unusual 
development of the root system. 
Plants able to live under extremely 
dry conditions are known as aero- 
phytes. Familiar examples of these are century plants and cacti. 
Some xerophytes, as the cacti, have a rather widely spreading 
root system extending quite near the surface of the earth ; such 
a root system makes the most of every one of the infrequent 
