88 ABSORPTION OF FOOD-SALTS BY LAND-PLANTS. 



adherent to bits of gravel that several little stones, weighing I'S grms., were found 

 clinging to it when it was lifted. The gelatinous mass, resulting from the swelling- 

 up of the extei'nal 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 00006 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 5,0 onward into the tissues 

 of a plant, but others pass out of the plant through the absorptive cells into 

 the earth. Amongst these eKminated substances, carbonic acid, in particular, 

 plays an important part. A portion of the earth-particles adhering to root-hairs is 

 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 efiect 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 

 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 upw^ards. 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 reflexuTn) is cut 



