108 



TEXTBOOK OF PLANT PHYSIOLOGY 



Cell-Wall 



Cytoplasm 



eus 



Fig. 42. — A diagram of an adult plant cell. 



Arrows show the direction of osmotic 



pressure {after Sinnott). 



a definite shape, it readily fills all hollows and projections of the 

 outer wall. 



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 nutri- 

 ent 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 the plant organism is that 

 the animal cells, amoebae, and 

 infusoria, do not have a rigid 

 wall, and consequently are 

 able to absorb and digest parti- 

 cles 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 molecules, or 

 even 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 a liquid than in the gaseous 

 state, the physiological property is the same. The particles of the 

 dissolved substance tend to be uniformly distributed in the avail- 

 able space; the volume of the solvent. In other words, dissolved 

 substances exhibit the phenomenon of diffusion. The slowness of 

 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 it will spread through the solvent. The large particles 



