CHAPTER VI 



THE MOVEMENTS OF WATER 



SECTION 33. General view. 



A PLANT can only live when it obtains a sufficient supply of water, with 

 which all its parts are saturated, and which in turgescent tissues generally 

 forms 60 to 90 per cent, of the entire weight. The amount of water present 

 may vary greatly, and a decrease is indicated by flaccidity and withering, 

 which if not checked is ultimately followed in most cases by death. 

 Seeds, mosses, lichens, &c., which are able to withstand desiccation, are first 

 awakened to new activity when they are supplied with water, and become 

 turgid. Variations of turgidity are always induced by the gain and loss 

 of water, and in terrestrial plants water must pass . from the subterranean 

 root-system to the transpiring organs in order to compensate for the loss by 

 transpiration. The amount of water which passes in this manner through 

 a terrestrial plant is in general very much greater than the amount present 

 at any given moment, while in comparison with the former the amount of 

 water which escapes when a vine stem bleeds is but trifling, as is also the 

 amount which is used in metabolism as a source of oxygen and hydrogen. 



The ways and means by which water is absorbed have already been 

 discussed (Sects. 25-27), and we are therefore at present concerned only 

 with the phenomena connected with the transport of water from one part 

 to another. The water-current is mainly employed to make good the loss 

 by transpiration, a function which, as has already been indicated in con- 

 nexion with the movements of gases (Chap. V), is more or less markedly 

 limited and regulated by the structural relationships and inherent pecu- 

 liarities exhibited by particular plants. A certain limitation is necessary 

 in order to maintain the proper relationship between the loss and the 

 supply. Where the climate and habitat are such that temporary periods of 

 drought occur, plants which are killed by desiccation can only survive the 

 unfavourable periods by economizing as far as possible their store of water. 

 The various means by which this end is attained cannot be discussed here ; 

 nevertheless, it is well known to how great an extent the adaption to a dry 



