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 

 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 xero- 



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 



FIG. 19. Effect of deficient 

 water supply on growth 



The plant is groundsel, a common 

 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" 



