70 PRACTICAL COURSE IN BOTANY 
strong affinity for water, and its presence or absence has a 
marked influence on their direction of growth, being often 
sufficient to overcome that of geotropism (Exp. 47). There 
are many trees and shrubs, notably willow, sweet bay, red 
birch, and the like, that grow best on the banks of streams 
and ponds, where their roots can have direct access to water. 
Excess of moisture, however, is injurious to most land plants 
by preventing the roots from getting sufficient air for res- 
piration. 
72. The conditions of absorption. — The sap in the root 
cells is normally denser than the water in the soil, so there is 
a continuous flow from the latter to the former. But if, 
for any reason, the density of the liquids should be reversed, 
the flow would set in the opposite direction, and if continued 
long enough, the strength of the plant would be literally 
‘“ sapped ”’ by the exhaustion of its tissues, so that it would 
die. What is this process of cell exhaustion called? 
73. The use of acid secretions to the root. — It was 
shown in Exp. 50 that carbon dioxide and probably other sub- 
stances occur in the im- 
mediate vicinity of roots. 
sie or - Carbon dioxide is an ac- 
: tive agent in dissolving 
Song the various mineral mat- 
_ ee et on Gal ters contained in the soil, 
Feo and as these last can be 
s ; absorbed only in a liquid 
— i Se ae , is _. Wy or a gaseous state (63), 
iin ‘ the advantage to the 
root as an absorbent or- 
gan, of being able to se- 
crete such active sol- 
vents, is obvious. 
74. Relation of roots 
Fie. 85. — A natural root etching, to the soil. — In order to 
found on a piece of slate. perform their work of ab- 
ne 
