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, 2.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 c.m. above the earth, the roots growing towards the ground 
develop their root-hairs 2 ¢.m. from their place of origin. But if the shoot is at a 
distance of 10 ¢.m., the roots only develop their root-hairs when they have attained a 
length of 10 em. 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 
offer 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 direction 
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 hes 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 afford supplies of nutritious material, are strikingly exhibited, also, 
by epiphytes growing on the bark of trees, such as tropical orchids and 
Bromeliacee; and again by plants parasitic on the branches of trees, of which the 
Mistletoe and other members of the Loranthacew afford examples. Although the 
absorption of food by these plants will not be thoroughly discussed till a later 
