GROWTH 173 



ume, but this does not necessarily mean growth, as we 

 have already seen; and again, if the cell is stretched, if 

 its volume is increased, by other means, its turgor must 

 either keep pace with this increase in volume, or fall. If it 

 keeps pace, because of the composition of the cell-sap and 

 of the abundance of water to be absorbed, it will be im- 

 possible to determine whether evident growth is dependent 

 upon turgor and is regulated by it, or not. If the turgor 

 fall during the increase in volume, it must be shown that 

 this fall is due to no other cause. On this point decisive 

 experimental evidence is still wanting. 



From experiments by True* on the different rates of 

 elongation in the roots of seedlings grown in water-culture 

 and suddenly transferred to culture media of higher or 

 lower density, it would appear "that growth and turgor- 

 pressure here stand in no directly proportional relation to 

 each other." Furthermore, Pfefferf has shown, in a case 

 where turgor would be at least equal!}' helpful, namely, 

 in the formation of cell-wall, that it is not necessary. 

 The question resolves itself then into this : is turgor- 

 pressure, so useful and so necessary in maintaining the 

 form of cells, tissues, organs, and organisms the force by 

 which increase in volume is attained, or only the means by 

 which increased volume is maintained? Apparently the 

 latter is more likely to be the case ; but if this is true, what 

 is the force by which the living protoplasm expands, and 

 by which it stretches its bounding walls? If turgor is not 

 the force by which visible growth is accomplished, then 

 the increase in the amount of water and in the volume of 

 cell-sap in the growing part is only the evidence, not the 

 intrinsic quality, of visible growth. After all, we are forced 

 to confess that the physiologist's knowledge of the forces by 

 which the living protoplasm works is very incomplete. 



Room is needed. Without it growth cannot take place. 



* True. R. H. On the influence of sudden changes of turgor and of 

 temperature on growth. Annals of Botany, vol. 9, 1895. 



t Pfeffer. W. Druck und Arbeitsleistung durch wachsende Pflanzen. 

 Abhandlungen d. K. Sachs. Gesellsch. f. Wissensch., Bd. XX., Heft 3. p. 

 429. 1893. 



