DIV. ii PHYSIOLOGY 235 



least resistance (p. 114). In the phenomenon of bleeding, also, water is forced from 

 parenchymatous cells into the cavities of the vessels ; although this process takes 

 place especially often in roots, it is not always absent in the cases of stems and 

 leaves. 



Thus, when fully analysed, all the phenomena described show a one-sided 

 excretion of fluid from living cells. That this does not always result from the 

 same cause is indicated by what was stated above, since the fluid is sometimes 

 nearly pure water, at other times more or less concentrated sap. 



1. The conceptions which have been formed regarding the one-sided excretion 

 of pure water from a cell cannot readily be summarised here. 



2. When the excreted fluid contains dissolved substances in considerable 

 quantity, for example in nectaries, two possibilities present themselves. Either 

 these substances come from within the cell and the protoplasm must have become 

 permeable in one direction to them ; or they have been formed from the outer 

 layers of the wall and withdrawn water from the cell sap osmotically. It appears 

 that both possibilities are realised. 



Conduction of Water ( 18 ) 



The water, which is partly given off in the form of vapour, 

 especially from the leaves, and in part exudes in the liquid form from 

 hydathodes and wounds, has, as a rule, been absorbed by the roots. 

 It has thus to traverse a path which, even in annual plants, may 

 amount to some metres, and in the giants of the vegetable kingdom 

 may be more than 100 m. ; the stems of Eucalyptus amygdalina 

 are 100 m., those of Sequoia gigantea 95 m. in height. Osmotic 

 passage from cell to cell would bring about the movement of this 

 water far too slowly to cover the loss. The movement of water for 

 this purpose, or, as it is called, the TRANSPIRATION STREAM, is prac- 

 tically confined to the woody portion of the vascular bundles, e.g. the 

 wood of trees. This is shown by a classical experiment repre- 

 sented in Fig. 244. At Z in the branch b all the tissues external 

 to the slender column of wood have been removed. Since the leaves 

 of this branch remain as fresh as those of the branch c, it is evident 

 that the transpiration current must pass through the wood and not 

 through the cortical tissues. On the other hand, when a short length 

 of the wood is removed from a stem without at the same time unduly 

 destroying the continuity of the bark, the leaves above the point of 

 removal will droop as quickly as in a twig cut off from the stem. 

 This experiment can be performed either on intact plants or on cut- 

 off branches placed in water ; the latter for a time, until changes 

 have taken place at the cut surface, absorb water as actively as 

 does the intact plant by its roots. When a branch is cut off and the 

 cut surface is placed in a solution of gelatine, which penetrates for 

 some distance into the vessels and can then be allowed to solidify, 

 the wood will be found to have lost its power of conducting water. 



