THE WATER-RELATION 



95 



lalls or recovers with it. Their tLirgor is also a necessary condition for 

 Growth to take place ; for it follows on the yielding of the resistant 

 cell-wall before the internal tension, a permanent increase in size of 

 the cell being the result. Thus adequate water-supply is essential 

 for active vitality. But obviously a deficiency in moderate degree 

 does not put a term on life, as the recovery of the wilted shoot 

 shows. 



In the case of any average Land-Plant, growing under average 

 conditions, it is rarely that the water-relation presents any extreme 



i?-'-,4--^^ 



Flo. 71. 

 Succulent stem and flowers of a Cactus, {.\fter Figuier.) 



difficulty. A supply meeting the demand is usually available, and 

 even herbaceous plants remain firm and do not wither beyond recovery 

 under apparent drought and wind. But in plants which live in dry 

 climates, or in positions where the water supply is apt to be deficient — 

 such as sand, or on rock-faces where little or no soil is present ; or in 

 the case of plants which grow perched upon the branches of others 

 (epiphytes), and thus have no direct access to the soil — the little 

 water that is available must be used economically. If withering or 

 actual drying up is to be avoided, special protection against undue loss 



