PLASMOLYSIS 133 



will j)ass iVom \\\v (■ell, caiisino the cell snh.staiur to shrink. But 

 the cell icall docs not shrink and a space is left between the contents 

 and the container, so rendering the latter apparent. This process 

 is termed plasmolysis. Free cells like red blood corpuscles may be 

 submitted to experiments similar to the plasmolytic one detailed 

 above. If corpuscles are put into a solution of" lower osmotic 

 pressure (hypotonic solution) than their contents, they will swell 

 up because of the passage inwards of water, i.e. endosmosis, and 

 will j)robably burst. This is called haemolysis, and luay be brought 

 about in other ways, which are, however, all obviously methods 

 for destroying a membrane (page 318). Artificial membranes may 

 be made which act in a way similar to animal cell coverings. 



(iii.) The work of A. V. Hill, Meyerhof and others, which has in 

 recent years done so much to elucidate that process so essential for 

 our well-being, viz. muscular contraction, has also pointed to the 

 necessity for postulating membranes in and on the contracting 

 imits (Chap. XIV.). These experiments, together wdth the fact 

 that it is quite impossible to conceive of energy changes taking 

 place in naked protoplasm, are sufficient evidence of the need for 

 cell membranes. 



Nature of the Membrane. 



The cell w^all differs considerably from the cell contents in 

 chemical composition and in physical state. In plants, it generally 

 consists of cellulose. Certain animals develop an exoskeleton of the 

 excreted salt of lime, of silica or of chitin. These excreted mem- 

 branes should not be confounded with the true cell membrane or 

 plasmahaut, which term connotes the layer of cell protoplasm 

 which, in animal and plant cells alike, lies between cell and environ- 

 ment. Only through this layer or membrane can the cell be 

 influenced by changes in the surrounding medium. Animal cell 

 membranes are more elastic than those of plants. Microscopic 

 examination shows generally a difference in refractive index round 

 the border of cells. 



Much research and nuieh speculation has been published on the 

 nature of these membranes. What conditions have they to 

 fulfil ? At least four qualities are essential : 



(1) The membrane must prevent the outward passage of cell 



substance while allowing water to pass freely in and out. 



(2) The membrane must permit of the intake of nutrient material 



and of the output of undigested and non-utilised material. 



(3) The w^aste products of metabolism,, gaseous and liquid, must 



find a way out, while oxygen must find a way in. 



(4) Finally, the mend^rane nmst be of such a nature as to allow 



