368 EFFECT OF DENSITY OF THE MEDIUM [Cn. XIII 



dation in the rate of growth. When, on the other hand, the 

 plant was transferred from a salt solution to which it had 

 become accustomed to pure water, a mechanical elongation 

 quickly occurred, but this too was followed by a retardation in 

 the rate of growth. Thus the reduction in the rate of growth 

 is not a mechanical result of change of medium, but is a 

 response to the stimulus of changed environment. 



We have hitherto considered almost exclusively the effect of 

 aqueous solutions. We must now consider how a variation in 

 the pressure of the atmosphere affects the growth of plants. 

 Upon this subject experiments have been made by JACCARD 

 ('93), who found that when the pressure of the air was reduced 

 from 78 cm. to between 10 cm. and 40 cm. of mercury, growth 

 is two or three or even six times as rapid as in ordinary air. 

 Likewise when the air is compressed to between three and six 

 atmospheres acceleration in growth occurs, although not to the 

 same extent as in the depressed air. If, however, the rarefac- 

 tion is very great (below 10 cm.) or the pressure excessive 

 (over 8 atmospheres), growth is retarded. Experiments indi- 

 cate that this result is not wholly due to the concentration of 

 the oxygen in the air. We may therefore conclude that a 

 change in pressure from the normal accelerates growth by 

 irritating the growing plasma up to a certain limit, beyond 

 which its injurious effects counterbalance its favorable ones. 



Summing up this account of the effect of concentrated solu- 

 tions upon the growth of organisms, we find that in general as 

 the density increases beyond the normal' the rate of growth 

 diminishes until, at a certain concentration, it ceases. In the 

 particular case of marine organisms a reduction in concentration 

 to a certain point causes excess of growth ; below that point, 

 diminution. It is probable that the diminution in growth is 

 proportional to the osmotic action of the medium (Chapter III). 



An explanation of the foregoing phenomena may be attained 

 by reference to the principles laid down in preceding chapters. 

 In Chapter X it has been shown that growth depends very 

 largely upon the specific imbibition of water. We do not 

 know the cause of the difference in imbibitory properties at 

 different times ; but if, as has been suggested, it is purely an 

 endosmotic phenomenon resulting from the secretion of plant 



