ABSORPTION OF FOOD-SALTS BY LAND-PLANTS. 89 



off and suspended in the air by a string. Whether it hangs with the apex upper- 

 most, i.e. in the position in which it grew naturally, or with the apex towards the 

 ground, it always, in a short space of time, produces roots which spring from the 

 axis between the fleshy foliage-leaves and bending sharply grow to the earth. Thus 

 in the former case their direction is contrary to the apex of the shoot; in the latter, 

 curiously enough, it is in the same direction. If the height at which the shoot is 

 suspended is only 2 cm. above the earth, the roots growing towards the ground 

 develop their root-hairs 2 cm. from their place of origin. But if the shoot is at a 

 distance of 10 cm., the roots only develop their root-hairs when they have attained a 

 length of 10 cm. The rule is, therefore, for the roots to grow until they reach the 

 nutrient soil without developing absorption-cells, and only to provide themselves 

 with them when they are in the earth. It is to be observed that these roots are 

 produced on the suspended shoot at places where, under normal conditions (i.e., if 

 the shoot were not cut off" and hung up), no roots would be developed. Subject 

 to abnormal conditions and liable to starvation, the plant sends out these roots for 

 self-preservation. 



Phenomena of this kind force one to conclude that a plant discerns places which 

 ofler a supply of nutriment, and then throws out anchors for safety to those places. 

 This power of detection may, undoubtedly, be explained by the influence which 

 conditions of moisture, in addition to the action of gravitation, have on the dii-ection 

 taken by growing roots. The root-hairs can only obtain food-salts when the ground 

 is thoroughly moist; and whenever roots, or rather their branches, have to choose 

 between two regions, one of which is dry and the other wet, they invariably turn 

 towards the latter. If seeds of the garden-cress are placed on the face of a wall of 

 clay which is kept moist, the rootlets, after bursting out of the seeds, grow at first 

 downwards, but later they enter the wall in a lateral direction. The longitudinal 

 growth of the roots is greater on the dry side than on the wet side, and this results 

 in a bending of the whole towards the source of moisture, in this instance the damp 

 wall. It has been established that the tip of a rootlet is very sensitive to the 

 presence of moisture in the environment. Where there is a moist stratum on one 

 side and a dry stratum on the other, a root-tip receives a stimulus from the unequal 

 conditions in respect of moisture; the stimulus is propagated to the growing part of 

 the root, which lies behind the tip, and the result is a curvature of the root towards 

 the moist side. Thus, the presence of absorbable nutriment, or rather of moisture, 

 in the ground explains the divergence of roots from the direction prescribed by 

 gravity. 



The extent to which the direction taken by roots in their search for food is 

 dependent upon the presence of that food, and the fact that roots grow towards 

 places that aflbrd supplies of nutritious material, are strikingly exhibited, also, 

 by epiphytes growing on the bark of trees, such as tropical orchids and 

 Bromeliaceoe ; and again by plants parasitic on the branches of trees, of which tlie 

 Mistletoe and other members of the Loranthacece afford examples. Although the 

 absorption of food by these plants will not be thoroughly discussed till a later 



