CYTOPLASM AND NUCLEUS 



protoplasmatic threads and lamellae become finer 

 and rarer. Only along the cell wall a thick 

 polioplasmatic layer persists. At last, when the cell 

 has nearly reached the definite size, we see, as a 

 rule, only the polioplasma layer along the cell wall, 

 surrounding one large vacuole which occupies the 

 whole central space of the cell. Even the nucleus, 

 formerly suspended on numerous fine plasmatic 

 threads and lamellae in the middle of the cell, is now 

 situated in the plasma layer near the wall, forming 

 a protuberance in this layer. The general im- 

 pression is that the mass of protoplasm does not 

 increase when cells are growing in length and 

 diameter. The nucleus even looks a little smaller 

 in adult cells than in young ones. Further, the 

 protoplasma must take up a considerable quantity 

 of water to form the vacuoles and to fill them with 

 a watery solution of different substances, which 

 solution is known as cell sap. Doubtless the 

 mechanism employed in forming vacuoles is con- 

 nected with the mechanism of growth. The whole 

 bulk of polioplasma does not swell when cells are 

 growing. The quantity of water in polioplasma it- 

 self seems to remain constant during the formation 

 of cell sap in the vacuoles. 



It is noteworthy that the polioplasma remains 

 pressed against the cell wall. Loss of water im- 

 mediately disturbs this normal state. Leaves, 

 when withering, lose their normal elastic and firm 

 condition, and at once their capacity of growth. 



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