106 PLANT PHYSIOLOGY 



body the water so strongly held on the surfaces of the soil par- 

 ticles. In California, the only water upon which most plants 

 not subjected to irrigation can draw during the greater part 

 of the dry season is that held hygroscopically, and that plants 

 grow at all or even survive during the dry season is positive 

 evidence that they do exert such an attractive force. What 

 are the means at hand? To answer this question we must 

 consider the physical properties of vegetable cells. 



The typical vegetable cell — an alga, a root-hair, a paren- 

 chyma cell — is bounded by a thin cellulose membrane per- 

 meated with water holding in solution a variety of mineral 

 and other substances. This membrane is firm, strong, 

 elastic, and is not only permeated with water, i. e. has 

 molecules of water between its molecules or groups of mole- 

 cules, but also permits the movement of molecules of water 

 and of substances in solution in water in and through itself. 

 The movement of water and of aqueous solutions through 

 a membrane is known as osmosis. Lining the cellulose 

 wall is the layer of living protoplasm which produced it. 

 The layer of protoplasm not only varies in thickness, being 

 thickest in young and thinnest in old cells, but is never 

 homogeneous. Apart from the nucleus, chromatophores, 

 and granules of food and other substances contained in it, 

 the surface of the protoplasmic layer in contact with the 

 cell-wall is differentiated into an exceedingly thin living 

 membrane, the ectoplast. Within the living protoplasm are 

 the vacuoles — one or more bodies of water holding in solu- 

 tion a great variety of compounds, organic and inorganic 

 — and the nucleus, also bounded, like the living protoplasm 

 against the cell- wall, by cytoplasmic membranes. The mem- 

 branes bounding the vacuoles are called tonoplasts. The 

 solution filling the vacuoles and permeating the cell, the cell- 

 sap, is the active agent in absorption, although its composi- 

 tion, and therefore its action, are controlled by the living pro- 

 toplasm, either by the substances formed by the protoplasm 

 and transferred to the cell-sap, or by the substances permit- 

 ted by- the cytoplasmic membranes to enter or pass out of the 

 cell, the vacuoles, or the nucleus. That the cytoplasmic 

 membranes do exert a controlling power over substances in 



