GENERAL CYTOPLASM 5 



protoplast could not again swell up on the replacement of the 

 salt solution by water if its osmotic substances had been lost. 

 This guardianship of the exchange of materials between the 

 the cells or between the cells and the external world is one of 

 the well-recognized functions of the plasma membrane. Since 

 the plasma membrane lies at the surface of the protoplast it 

 must receive and transmit to the other parts the stimuli that 

 come from without. Until the cell-wall is built the plasma 

 membrane doubtless aflfords some rigidity and protection to 

 the parts within; and when the time for the building of the cell- 

 wall arrives it seems that the plasma membrane constructs it 

 by the chemical transformation of its own substance into the 

 substance of the wall. 



General Cytoplasm. — The cytoplasm is the living matrix 

 in which the nucleus and plastids are imbedded. In very young 

 cells it fills out all of the space not occupied by the nucleus and 

 plastids (Fig. i, A), but in old cells it becomes a very thin film, 

 hardly greater than .0006 mm. in thickness lining the cell-wall. 

 In cells that have been killed, fixed and stained in the usual ways 

 (see chapter on The Preparation of Sections) the cytoplasm has 

 a spongy or netted appearance (Fig. i. A). 



It is uncertain whether the cytoplasm is really sponge-like 

 with irregular and intercommunicating canals or alveolar with 

 each cavity a closed sac. Whatever the exact character of the 

 cavities may be, they are filled with cell-sap or, in many in- 

 stances, with insoluble reserve food, such as starch, proteids, 

 and oils, and excretions, such as crystals of calcium oxalate. 



As has been stated, as the cell grows older some of the cavi- 

 ties in the cytoplasm enlarge and coalesce, and are then known 

 as vacuoles. A plasma membrane is formed about the vacu- 

 oles similar to the exterior plasma membrane already described, 

 and it exercises a selective function over the passage of mate- 

 rials to and from the vacuole just as does the e.xterior mem- 

 brane to and from the protoplast as a whole. The spongy, 

 or alveolar, condition of the cytoplasm persists until the divi- 

 sion of the nucleus preparatory to cell division sets in. when a 



