5i-i EFFECTS OF GROWING CELLS ON ENVIUONMENT. 



into the tiny crevices of stone, tliey crack and crumljle the permeated substi-atum 

 not only by lateral pressure, but they act also lever- wise, and vigorously press 

 up the shattered particles. The absorbent cells or rhizoids of mosses and liver- 

 worts also exercise a like action on their substratum, and this is maintained, as in 

 the lichens, essentially by the fact that substances are excreted from the growing 

 cells by which the substratum is partially converted into soluble compounds. 

 Moreover, the pressure which these delicate cells exert on the substratum by 

 their growth may be demonstrated by experiment. If liverworts are laid upon 

 damp folded filter -paper in a space saturated with vapour, in forty -eight hours 

 they will send out rhizoids which grow through the paper. The holes in which 

 the cells of the rhizoids are now seen certainly did not previously exist in the 

 paper. The felt of threads in the filter-paper is so dense that starch-grains of 

 maize, having a diameter of only about 2 thousandths of a millimetre, cannot find 

 sufficient space to slip through, and thus still less can the rhizoids of liverworts 

 penetrate the felt, as these have a diameter of from 10 to 35 thousandths. The 

 holes must, therefore, be first formed by the growing cells of the rhizoids. The 

 threads of the felt must be powerfully driven asunder, and this requires at any 

 rate a comparatively large expenditure of force. 



The hyphal threads of a mushroom, which unite to form dense fructifications 

 and grow up from the subterranean mycelium in a comparatively short time, often 

 raise considerable pieces of earth, and the cap-shaped fructifications of Lactarius 

 scrohiculatus, Agaricus vellereus, and Hydnum repandum are indeed frequently 

 thickly covered with larger and smaller fragments of earth, raised by them during 

 their upward growth. An instance is also known in which a stone of 160 kg. 

 was raised and shifted by the gi'owing fructification of a fungus of the mushroom 

 tribe. 



Nor is the pressure which the growing cells of flowering plants exert on their 

 environment less considerable. The absorbent cells of roots embedded in the earth, 

 which are called root-hairs, appear fairly straight, although the spaces between the 

 particles of soil filled with air and water are certainly not rectilinear. It cannot be 

 doubted, therefore, that the root-hairs in spite of their delicacy, nevertheless push 

 the small particles of earth on one side, and in their growth follow, as nearly as 

 possible, a straight course. The apices of the main roots of flowering plants, when 

 they grow downwards, form actual channels by pressure on their environment, 

 pushing the portions of soil powerfully asunder, and penetrating into the ground 

 like a gimlet. And it would be a mistake to suppose that they are only drawn 

 downwards by gravity. The roots of bean-seeds which have been germinated in a 

 layer of water spread above quicksilver actually penetrate into the quicksilver. It 

 has often been noticed that the roots of trees which have reached fissures in walls 

 or clefts of rocks are able to shatter the walls and to crack the stone by their 

 further thickening. One at least out of the great number of instances may here be 

 noticed. On either side of the little Tyrolese Gschnitz-thal are terraces strewn with 

 large blocks of stone, which are considered ancient diluvial moraines. The blocks 



