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1897 | THE CURVATURE OF ROOTS 3I7 
affecting the shoot. Water and food substances lie below the 
surface of the substratum, and the root has developed a highly 
marked form of geotropism, which enables it to penetrate the 
soil. Water and food substances, however, are by no means so 
uniformly distributed as sunlight and carbon dioxide. While 
water exhibits a fairly horizontal distribution in quantity, yet so 
far as its actual availability is concerned differences correspond- 
ing to the physical characteristics of the soil are to be found. 
The vertical distribution is modified in the same manner. The 
mineral food substances present no system or uniformity of dis- 
tribution whatever. As a matter of fact the masses of food sub- 
stances may and do lie in all possible directions from the absorb- 
ent zone of the apical portion of the root. In order to reach 
such irregularly distributed masses of nutritive substances it is 
evidently necessary that the root should develop an irritability 
to a much greater number of forces than any member or organ 
of the shoot, and furthermore it is evident that all the forms of 
irritability thus acquired must be located in the apical portion 
of the root, the proper directive activity of which only is con- 
cerned with the absorptive processes. The coincidence of sev- 
eral forms of irritability within such narrow limits has necessi- 
tated differentiations in another direction from that offered by 
the shoot. The differentiation of the shoot resulted in a tend- 
€ncy to separate the different forms of irritability with their 
attendant mechanisms. The increase of the efficiency of the root 
has resulted in the acquisition of a constantly increasing number 
of forms of irritability, within a limited mass of tissue, the mech- 
anism of which must necessarily be identical. Still further this 
has resulted, of course, in the differentiation of the separate parts 
of the mechanism and increase of its delicacy of reaction. This 
may be held to apply to all similar arrangements, especially in 
the ecological reactions shown by the so-called “sensitive” plants. 
V. IRRITABLE ORGANIZATION OF THE ROOT. 
On account of the fact that the irritable mechanism of roots 
‘is located in the embryonic region of the organ, no distinct 
