PHYSIOLOGY 



is even more remarkable ; living in a medium containing 3 per 

 cent of common salt, and but little potassium salts, they neverthe- 

 less accumulate much larger quantities of potassium than sodium. 

 In addition they store up phosphates, nitrates, and iodine, sub- 

 stances which are all present in sea-water in such small quantities 

 as scarcely to be detected by chemical analysis. Penicillium glaucum 

 is able to grow on a nutritive solution containing 21 per cent of 

 sulphate of copper, owing to the power it possesses of allowing the 

 entrance to the cell of the salts it requires, while preventing that of 

 the copper sulphate ( 20 ). 



That osmosis may continue into a cell, it is essential that the 

 absorbed material should become transformed into something else, 

 either by the activity of the protoplasm or by some other means. Local 

 accumulations of sugar or other soluble reserve material in fruits, 

 seeds, bulbs, and tubers would otherwise not be possible ; for osmotic 

 action, if undisturbed, must in the end lead to the uniform distribution 

 of the diffusible substances equally throughout all the cells. But if 

 equilibrium is prevented by the transformation of the diffusible sub- 

 stances into others that are indiff'usible, the osmotic currents towards 

 the transforming cells will continue, and the altered and no longer 

 diffusible substances will be accumulated in them. In this manner 

 glucose passing by osmosis into the cells of tubers or seeds becomes 

 converted into the insoluble polysaccharide, starch. As a result of 

 this a constant movement of new glucose is maintained towards these 

 cells, which thus become reservoirs of accumulated reserve material 



Water and Mineral Substances 

 Without water there can be no life. THE LIVING PORTIONS OF 



ALL ORGANISMS ARE PERMEATED WITH WATER; it is Ollly when ill this 



condition that their vital processes can be carried on. Protoplasm, 

 the real physical basis of life, is, when living, of a viscous, thinly 

 fluid consistency, and when dried either dies or becomes inactive. 



Protoplasm, when in a state of inactivity, as in spores and seeds, can often 

 endure a certain degree of desiccation for a limited time. During such periods its 

 actual vital functions cease, and only renew their activity when water is again 

 supplied. 



With the exception of some succulent plants which are uninjured by the loss 

 of nine-tenths of their water, plants as a rule have their activity impaired by the 

 loss of water iu withering and are killed by complete desiccation. It is always to lie 

 regarded as due to some special provision or exceptional quality when entire plants 

 or their reproductive bodies which have been dried can be again brought to life 

 by a supply of water. Thus, for example, some Algerian species of Isoetes and the 

 Central American Selaginclla lejtidophylla can withstand droughts of many months' 

 duration, and on the first rain again burst into life and renew their growth. In 

 like manner many Mosses, Liverworts, Lichens, and Algae growing on bare rocks, 

 tree-trunks, etc., seem able to sustain long seasons of drought without injurv. 



