190 BOTANY part i 



ensue if the substances that are used up are not at least replaced 

 in the process of nutrition. 



In the following account the absorption of food-materials will first 

 be dealt with, and then their utilisation. Those substances which in 

 the higher plants are obtained from the soil will ])e considered in the 

 first place, and then those which are derived from the atmosphere. 



2. The Food-Materials ; their Absorption and their Translocation 



in the Plant 



(a) Water 



Without water there can be no life. TiiE LIVING portions of 

 ALL organisms ARE PERMEATED WITH WATER ; it is only when in 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. 



AVith 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 in witliering, and are killed by complete desiccation. It is always to be 

 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 supi^ly of water. Thus, for example, some Algerian species oi Isoeies and the 

 Central American Selagindla Icj'idophylla 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 al)le to sustain long seasons of drought without injury. 

 Seeds and spores, after separation from their parent plants, remain productive for 

 a long time ; seeds of Nehcmbium, which had been kept dry for over one hundred 

 years, proved capable of germination. A similar vitality was shown by moss 

 spores which had lain in a herbarium fifty years. The often-repeated assertion 

 concerning the germination of wheat found with Egyptian mummies ("mummy- 

 wheat ") has, however, been shown to be erroneous. Many seeds lose their power 

 of germination after having been kept dry for only one or a few years ; others, 

 even after a few days ; and others again cannot endure drying at all. On the 

 other hand, the seeds of some water plants {Eichhornia, etc.) germinate better 

 after being dried for a period. It must not be forgotten that in all these instances 

 a certain amount (about 9-14 per cent) of hygroscopic water is retained by plants 

 even when the air is quite dry. Over the sulpluiric acid of the desiccator seeds 

 retain for weeks 6 per cent or more of their weight of water. Even drying at 110° 

 or the action of absolute alcohol, can be borne by some spores and seeds. 



Apart from permeating and energising the cells, water has other 

 and more varied uses in plant life. It is not only indirectly indis- 

 pensable for the solution and transport of the products of metabolism, 



