180 PLANT PHYSIOLOGY 



Each of the component cells of a multicellular tissue, organ, 

 or organism, is limited in all its behavior by the cells which 

 surround it. The single cell of the unicellular organism is 

 not so limited, being constrained only by the conditions 

 prevailing in itself and in its lifeless surroundings. 



Attempts have been made to attribute the maximum size 

 ordinarily attained by organisms to one or two of the three 

 conditions named on page 168 as making growth possible. 

 It is said that an organism or a cell cannot grow beyond 

 a certain size because there may not be room. If this be 

 true, then the word room must be used with a broader 

 meaning than that attached to it in our discussion on 

 pages 173 and 174 : it must mean, as stated on page 6, 

 freedom from interference of every sort. This last is un- 

 doubtedly true, for a plant with such an impulse to grow 

 that it might otherwise cover the whole earth would be 

 prevented by the presence, if not by the attacks, of the other 

 organisms living at the same time. So the individual is 

 kept within a certain size. 



Again, it is said that nutrition fixes the limit of growth. 

 In a growing spherical cell, the increase in surface and in 

 mass are to each other as the square to the cube, "in other 

 words, the smaller the cell, the greater is the surface in 

 proportion to the mass; and the more the cell grows, the 

 less does the surface grow in proportion to the mass."* It 

 is said that since all food-materials are taken in through 

 the surface, the supply of food will be insufficient when the 

 surface becomes too small in proportion to the mass. This 

 implies, however, that the absorbing power does not in- 

 crease proportionally with the mass. The absorbing power, 

 as we have seen, is the osmotic force exercised by the cell- 

 sap upon the solutions and the constituents of the solu- 

 tions outside the ceU. This osmotic force depends upon the 

 differences in the composition, absolute and proportional, of 

 coil-sap and surrounding liquid. The larger the volume of 

 cell-sap, the more slowly can it be made like the liquid out- 

 side, i. e. the more slowly will the osmotic force be dimin- 



* Verworn, M. Allgemeine Physiologie, Ite Aufl., p. 511, 1895. Engl, 

 transl. by Lee, General Physiology, p. 530, 1899. 



