PHYSIOLOGY 179 



of a viscous, thinly fluid consistency, and when freed from its water 

 either dies or becomes perfectly inactive. 



The circumstance that protoplasm, when in a state of inactivity, as in spores 

 and seeds, can often endure a certain degree of desiccation for a limited time, forms 

 no exception to this rule. During such periods its actual vital functions entirely 

 cease, and only renew their activity when water is again supplied. 



In most plants desiccation occasions death, and it is always to be regarded as 

 due to some special provision or exceptional quality when entire plants or their 

 reproductive bodies can be again brought to life by a subsequent supply of water. 

 Thus, for example, some Algerian species of Isoetes, and the Central American 

 Selaginella lep'idophijlla, can withstand droughts of many months' dui'ation, 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 injury. Seeds and 

 spores, after separation from their parent plants, remain productive for a, long 

 time ; seeds of Mimosa, which had been kept dry for over sixty years, proved as 

 capable of germination as those of recent growth. 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 a year ; others, even after a few 

 days ; and others again, as the seeds of the willow, cannot endure drying at all. It 

 must not be forgotten that in all these instances a certain amount of hygroscopic 

 water is retained by plants even when the air is quite dry. Over the sulphuric 

 acid of the desiccator, seeds retain for weeks 6 per cent or more of their weight of 

 water. The withdrawal of this hygroseopically absorbed water kills all vegetable 

 tissues without exception. 



Apart from permeating and energising the cells, water has other 

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

 pensable for the solution and transportation of the products of 

 metabolism, but also indirectly, in that its elements, hydrogen and 

 oxygen, are made use of in organic compounds in plant nutrition. 

 Water thus used (cf. p. 200) may be designated constitution water. 

 It is also necessary for the turgidity and consequent rigidity of paren- 

 chymatous cells (p. 165) ; it is of use in the process of the growth of 

 plant cells, which take it up in large quantities, and, through their 

 consequent expansion, enlarge their volume with but little expenditure 

 of organic substance. 



A further and still more important service which water performs 

 for plants consists in the conveyance and introduction into the 



PLANT BODY OE THE NUTRIENT SUBSTANCES OF THE SOIL. Although 



a large amount of water is retained in the plant body (up to 96 per 

 cent in succulent tissues) for the maintenance of rigidity and enlarge- 

 ment of the organs, a still larger quantity of the water taken up by 

 the roots passes through the plant merely as a medium for the trans- 

 port of nourishment, and is again discharged through the leaves by 



