28 FUNDAMENTALS OF FRUIT PRODUCTION 



Table 17. — The Influence of Temperature Upon Rate of Transpiration 



{After Burgerstein^^) 



Water Transpired per Hour (in Per Cent. 

 Average Temperature (Centigrade) . of Weight op Branch) 



- 2.0 0.288 



- 2.8 0.227 

 -•5.2 0.131 



- 5.7 0.127 



- 6.2 0,093 



- 6.8 0.028 

 -10.7 0.019 



THE WATER CONDUCTING SYSTEM OF THE TREE 



The conduction of water from the roots to the leaves has been investigated 

 by Dixon," from whom the following account is largely taken. The water 

 absorbed by the root hairs passes by osmosis from cell to cell through the cortex 

 of the root and into the cavities of the wood cells. It then passes through the 

 younger layers of wood from the roots through the trunk to the branches and 

 eventually to the tracheae of the leaf. From these cells it is abstracted by the 

 endodermis and eventually finds its way by osmosis to the mesophyll cells from 

 whose outer surfaces it evaporates. Water passes into the cavities of the wood 

 cells in the root by osmosis. The osmotic system involved here consists of the 

 water of the soil, the solution in the elements of the wood, and the cortical cells 

 of the roots which constitute the semi-permeable membrane separating these 

 two solutions. These cortical cells have a higher osmotic pressure than the 

 tracheae but, being fully distended, they function merely as a complex membrane 

 across which a flow of water takes place. The concentration of the solution 

 which fills the tracheae is higher than that of the soil solution and is maintained 

 by the secretion of sugars from the wood parenchyma cells with which they come 

 in contact.* This osmotic system may develop considerable pressure and result 

 in the exudation of Uquid from cut surfaces as in the well known phenomenon 

 of bleeding in the vine. 



After the water has entered the wood cells in the root, it is carried up in the 

 woody tissue in an unbroken column, which extends into the veinlets of the 

 leaf. The water in the conducting tracts of high trees hangs there by virtue of 

 cohesion. It must not enclose bubbles, which would break the column of 

 water and permanently interfere with conduction. The structure of woody 

 tissue may be considered a special adaptation which confers stability on the 

 transpiration stream and prevents bubbles which may develop from occupying 

 more than an infinitesimal part of the cross section of the whole water current. 

 The imbibitional properties of the walls of neighboring water-filled tracheae 

 render the water continuous between them. If a bubble develops anywhere in 

 the transpiration stream it will enlarge until it fills the cavity of the cell in which 

 it originated, but the walls of the tracheae limit the bubble and prevent its further 

 expansion. From these considerations it follows that the column of water 

 will not be broken unless a very large number of the conducting tubes contain 

 air. Despite its mobiUty the water, as it rises in the wood, behaves very much 

 like a rigid body. 



