258 TRANSFER OF WATER THROUGH THE PLANT. 



of water, and thus prevented from coming in contact with those 

 around it. According to this hypothesis, all the water in a cell- 

 wall is practically continuous, and can flow freely between the 

 micellae ; therefore, if a cell contains its maximum amount of 

 water, and the cell-wall is tense, the water is in a state of equi- 

 librium. Likewise in a tissue containing its maximum amount of 

 water this .is in equilibrium. But the balance can be easily dis- 

 turbed in a plant by evaporation from the surface, or by other 

 causes before mentioned. If, however, a sufficient part of the 

 absorbing surface of the plant is in contact with water, the bal- 

 ance can be restored, since the water in the cell- walls is practi- 

 cally continuous with that in the surroundings. The equilibrium 

 is restored by the transfer of the water outside the cell-wall to 

 the cell-wall itself, and thence to the parts within. The tendency 

 to the restoration of the equilibrium of water in a plant is so 

 great that root-hairs can abstract even the firmh* adherent hygro- 

 scopic water from particles of soil (see 644). From the roots or 

 other absorbing organs the water passes sooner or later to the 

 place of consumption. 



690. In most cellular plants and in masses of cellular tissue 

 all the cell-walls have substantially the same capacity for transfer 

 of water ; but in all plants which possess a fibro-vascular system 

 the transfer takes place chiefly by means of the lignified cell- 

 walls ; and even in cellular plants like mosses, it is in those cells 

 which are elongated and otherwise differentiated to form an im- 

 perfectly developed framework that the rapid transfer is made. 



691. Transfer of water in woody plants. In ligneous plants 

 the water is transferred most rapidly through the woody tissues. 

 This is experimentally proved by ' ' girdling " their stems ; that 

 is, removing a ring of bark without injuring the wood. For a 

 time the leaves remain fresh, and the plants appear to suffer 

 only slightly, if indeed at all. An early experiment in regard 

 to the transfer of water is that by Hales (in 1731), who says r 1 

 "' I cut off the bark, for one inch length, quite round a like 

 branch of the same oak ; eighteen days after the leaves were 

 as green as any on the same tree." Further experiments have 

 shown that the rapid transfer is made chiefly in the younger 

 wood of the stem, and not in the heart-wood ; and, also, that 

 the water is transferred most rapidly in the portions of new wood 

 having the coarser texture known as spring wood 2 (see 395). 



1 Statical Essays, i., 1731, p. 130. 



2 Sachs : Vorlesungen iiber Pflanzenphysiologie, 1882, p. 275. 



