1] UPON THE RATE OF GROWTH 375 



probably an increase in cross-section of the whole stem. There 

 is indeed considerable experimental evidence for this cofl^ 

 elusion. Thus SCHENCK ('93) found that when stems are 

 irritated by twisting or bending, an excessive growth both 

 of cell-walls and of the wood as a whole follows. So, too, 

 NEWCOMBE ('95) finds that roots become strengthened by 

 attaching weights to them. In fact, if stems are deprived of 

 their normal swaying movements, for instance by enclosing 

 internodes in plaster casts which inhibit lateral movements and 

 partly support the weight of the superior part of the plant, 

 their walls remain abnormally thin. , 



These effects of deformation are of especial importance 

 because they are so clearly not at all directly mechanical but 

 adaptive ; they are, indeed, rather opposed to the direct 

 mechanical effects which would tend to stretch the cells, and 

 thus to diminish the thickness of their walls. Here again the 

 organism shows itself a highly irritable thing, capable of 

 responding in an adaptive fashion. N 



4. Local Removal of Tissue. When a protist, for instance 

 a Stentor, is transsected, certain changes take place along the 

 cut surface. First, there is a warping .of the edges towards 

 each other ; secondly, rapid growth (differential growth, page 

 287) occurs. Similarly, if a Hydra be cut lengthwise, the free 

 edges may fold towards each other so as to form a smaller cyl- 

 inder, and the seam, by growth, will be healed over. So, too, 

 in the higher animals, the removal of a bit of tissue results usu- 

 ally in the closure of the wound and growth to fill the gap. 

 We may call these two processes warping and regenerative 

 growth. 



The causes of these two processes are probably different. 

 The warping seems to result from the presence of tensions and 

 pressures in the tissues whose equilibrium is disturbed by the 

 cut. This process is probably grossly mechanical. The regen- 

 erative growth, however, must have a less direct explanation. 

 It is apparently a typical response to the stimulus of cutting, 

 or of the new environment presented at the cut edge. 



Here, too, may be mentioned an experiment by LOEB ('92, 

 p. 51), which throws light upon the cause of these internal 

 tensions. Below the crown of tentacles of a Cerianthus a 



