THE CELL. 



first claim our attention. The water-bearing cavities (vacuoles) in- 

 crease more and more in size and subsequently come in contact and 

 become flattened by mutual pressure. Finally they are separated 

 only by thin plasmic membranes and threads ; when these break the 

 vacuoles flow together to form one. The plasm then lines the inner 

 surface of the cell- wall as a 

 membrane which is usually 

 very thin, but which is 

 never absent from the liv- 

 ing cell. This membrane 

 is called the primordial 

 utricle or plasmic utricle. 

 On account of its frequently 

 immeasurable thinness it is 

 invisible as long as it is 

 in contact with the cell- 

 wall. If by artificial means 

 the plasmic utricle can be 

 caused to separate from the 

 wall by contraction, then 

 this is looked upon as giv- 

 ing evidence that it was a 

 living cell. (Compare Fig. 



10 



The cell-wall and the 

 plasmic utricle, the two 

 coverings of the cell con- 

 tents, differ (1) chemically, 

 in that the primordial utri- 

 cle being a part of the plasm 

 is an albuminoid substance, 

 while the cell-wall belongs 

 to the group of carbohy- 

 drates and contains there- 

 fore C, H, and O, the latter 

 in the proportion to form 

 water (H 2 O) ; (2) physically, in that the cell-wall is highly elastic 

 with but little extensibility, while the plasmic utricle is very ex- 

 tensible and only slightly elastic. To this must be added a second 

 physical difference, that of diosmosis. The physical differences are 



fl 



' * oun parenchyma-cell of Zea Mays. 

 A normal; 5, plasmolyced. m, membrane; p and 

 h < protoplasmic utricle; n, nucleus; s, cell-lumen 



witb ^p. (After prank.) 





