CHAPTER III 



ABSORPTION AND TRANSMISSION OF SOLUTES 



WITH the exception of the naked amoeboid cells occur- 

 ring in certain stages of the life histories of a few fungi and 

 algae, together with the cell complexes constituting Myxo- 

 mycete plasmodia, plant cells are unable to engulf solid food. 

 The presence of the cellulose membrane makes this fact 

 very evident. In order to be absorbed into a cell, any sub- 

 stance must first be in the form of an aqueous solution. 

 Even where the process of engulfing takes place, the food 

 does not truly pass inside the protoplasmic body until it is 

 dissolved ; around each food body in a plasmodium is a 

 vacuole lined by a plasmic membrane which is probably 

 identical in nature and origin with that covering the exte- 

 rior of the protoplasmic mass. In such cases the food is 

 digested in this vacuole and the products of digestion are 

 absorbed through this membrane. 



I. ABSORPTION OF GASES 



There are, in general, two forms of material in solution 

 which are absorbed by the plant, namely, gases and solids. 

 As has been seen, gases enter into aqueous solution when 

 they are simply brought into contact with the solvent. All 

 the natural water on the surface of the earth contains in 

 solution oxygen, nitrogen, argon, carbon dioxid, etc. The 

 first and the last of these gases are the only ones which are 

 important in plant metabolism. The moist cellulose mem- 

 brane and the protoplasts are all permeable to these dis- 

 solved gases, and, being soluble in water, they will diffuse 

 wherever it can diffuse. Thus there must be a tendency to 

 equalize the diffusion tension of oxygen and carbon dioxid 



115 



