12 PLANT PHYSIOLOGY 



The properties of the walls delimiting the cell determine not 

 only the form of it but also the laws governing the penetration 

 of substances from the surrounding medium into the cell. These 

 laws, in their turn, determine the conditions of absorption of 

 nutrient substances by the cell. The presence of a rigid wall 

 around every cell prevents the absorption of solid food. It is 

 well known that one of the fundamental physiological differences 

 between the animal and plant organism is that the animal cells 

 (amoebae and infusoria) do not have a rigid wall and conse- 

 quently are able to absorb and digest particles of solid food, while 

 plant cells depend exclusively on substances dissolved in water. 



Substances dissolved in water are frequently conceived of as 

 being in a state approaching that of the gases. As has been 

 shown, they are molecularly dispersed. Their separate mole- 

 cules or ions are scattered among the molecules of the solvent. 

 They have lost the power of cohesion and are in a condition of 

 continual motion, similar to that of gas particles. Though this 

 movement is considerably slower in the liquid than in the gaseous 

 state, it leads to the same results. The particles of the dissolved 

 substance tend to be uniformly distributed in the available 

 space — the volume of the solvent. In other words, dissolved 

 substances exhibit the phenomenon of diffusion. The slowness 

 of the diffusion of such substances may be due to the rather high 

 density of the medium in which they move, as well as to the fact 

 that each of their molecules or ions is bound to the nearest water 

 particles, which form around them a shell of water. Otherwise, 

 the laws governing the diffusion of gases may be applied to 

 diffusion of liquids in the cell. The fundamental law that the 

 rate of diffusion is in inverse proportion to the size of the particles 

 is likewise applicable here. The lower the molecular weight of a 

 substance, the more rapidly will it spread through the solvent. 

 The large particles of a substance of colloidal character move 

 much slower than the molecules of crystalloids. 



If a diffusing substance meets a membrane in its path, diffusion 

 becomes more or less complicated. If this membrane is porous 

 and the pores are filled with the same solvent, then there is less 

 complication. In this case, however, the rate of diffusion is 

 still more reduced. Only the largest colloidal aggregates are 

 retained by the membrane. Membranes of this type are used 

 in dialysis, by means of which colloids may be separated from 



