i8 



THE PLANT CELL 



cytoplasm, vacuoles, and nuclear cavity. The powers of osmosis 

 and diffusion are at work here, essentially as we find them in 

 artificial cells and through artificial and lifeless membranes; but 

 the chemical work done by the living protoplast makes and 

 keeps up the conditions necessary to osmosis and diffusion, and 

 the plasma membranes modify, and to a certain extent regulate, 

 the osmotic and diffusion exchange of materials into and from 

 the body of the protoplast. 



Fig. 9. — I and 2, nuclei from the tapetal Plasmodium of the sporangium of Botrychium 

 Virginianum, i, in the usual form, and 2, flattened while entering a crevice between spore- 

 grouDs; 3, cell from Nitella showing rotation of the cytoplasm as indicated by the arrows; 

 4, cell from stamen-hair of Tradescantia, showing circulation of the cytoplasm as indicated 

 by the arrows. 



(b) Construction. — The protoplast builds complex substances 

 from simpler ones, as when sugar, starch, oil, and proteid foods 

 are built from carbon dioxide, water, and salts of nitrogen and 

 sulphur; and when the foods are assimilated to the substance 

 of the protoplast itself. 



(c) Destruction. — In respiration the protoplast breaks down 

 a part of its own substance, and probably also reserve foods, into 

 simpler compounds; and the self-destruction of the protoplast 

 appears to take place in the production of secretions, such as 

 cell-wall and digestive ferments. 



{d) Excretion. — Frequently substances that are of no use are 

 excluded from the protoplast by inclosure in an insoluble form 



