88 ABSORPTION OF FOOD-SALTS BY LAND-PLANTS. 
adherent to bits of gravel that several little stones, weighing 1°8 grms., were found 
clinging to it when it was lifted. The gelatinous mass, resulting from the swelling- 
up of the external coat of the cell, does not in any way hinder absorption or the 
passage of food-salts in solution. Nor does the inner coat, the thickness of which 
varies between 0°0006 m.m. and 0°01 m.m., constitute any impediment to imbibition. 
In addition to the absorption of nutritive salts by root-hairs, there is also, in 
many cases, an interchange of materials; that is to say, not only do substances 
infiltrate from the earth into the absorption-cells, and >o onward into the tissues 
of a plant, but others pass out of the plant through the absorptive cells into 
the earth. Amongst these eliminated substances, carbonic acid, in particular, 
plays an important part. A portion of the earth-particles adhering to root-hairs are 
decomposed by it, and food-salts in immediate proximity to those cells are hereby 
rendered available and pass into the plant by the shortest way. 
Having now seen that land-plants take in food-salts by means of special 
absorptive cells, it is natural to find that each of these plants develops its 
absorption-cells, projects them, and sets them to work at a place where there is 
a source of nutritive matter. The parts that bear absorptive cells will accord- 
ingly grow where there are food-salts and water, which is so necessary for their 
absorption. The Marchantias and fern prothalli spread themselves flat upon the 
ground, moulding themselves to its contour. From their under-surfaces they 
send down rhizoids with absorptive cells into the interstices of the soil. Roots 
provided with root-hairs behave similarly. If a foliage-leaf of the Pepper-plant 
or of a Begonia be cut up, and the pieces laid flat on damp earth, roots are 
formed from them in a very short time. The roots on each piece of leaf proceed 
from veins near the edge, which is turned away from the incident light, and 
grow vertically downwards into the ground. 
It is matter of common knowledge that roots which arise upon subterranean 
parts of stems, like those formed on parts above-ground, grow downward with a 
force not to be accounted for by their weight alone. This phenomenon, which is 
called positive geotropism, is looked upon as an effect of gravitation. The idea is 
that an impetus to growth is given by gravity to the root-tip, and that a trans- 
mission of this stimulus ensues to the zone behind the tip where the growth of the 
root takes place. It is noteworthy that if bits of willow twigs are inserted upside 
down in the earth, or in damp moss, the roots formed from them, chiefly on the shady 
side, after bursting through the bark, grow downwards in the moist ground, pushing 
Ss 
aside with considerable force the grains of earth which they encounter. The 
appearance of a willow branch thus reversed in the ground is all the more curious 
inasmuch as the shoots, which are developed simultaneously with roots from the 
leaf-buds, do not grow in the general direction of the buds and branches, but turn 
away immediately and bend upwards. Thus the direction of growth of roots and 
shoots produced on willow-cuttings remains always the same, whether the base or the 
top of the twig used as a cutting is inserted in the earth. A similar phenomenon is 
observed if the leafy rootless shoot of a succulent herb (e.g Sedum rerleeum) is cut 
