io8 



r\N\ « i| THE LIVING PLANT 



b u bbl< «'t bubbles would d< stroj i ohesion 



in the < olumns oi water, di» ontinuity tl veloping by the expan- 



n ,,, the bubble columns are subject to tension. 



It has »>• emarked that, in point ct, the water-columns 



lin the • 'i<ii include air-bubbles: but the upholders of 



the Cohesion Theory maintain that the contents are free from bubbles 



m .1 sufficient proportion of the vessels 

 to provide the necessary degree of con- 

 tinuity : moreover, that the effect of 

 the air-bubbles is only local. 



Any tension in the xylem vessels will 

 spread downwards into the roots, and 

 will there promote the flow of water into 

 the xylem from the adjacent living cells. 

 Tins will activate the osmotic chain which 

 across the cortex, and continued 

 absorption of water from the soil will 

 ensue (see Fig. 72, also p. 99). 



It appears, however, that there is an- 

 other mechanism capable of inducing 

 movement of water into the vessels of 

 the root and up through the plant. 

 This is suggested by the phenomena of 

 Exudation and of Root Pressure. If a 

 plant is severed a few inches above the 

 soil on a warm day, and the cut sur- 

 Fic. 74. face immediately examined with a lens, 



ggSj it will appear dry. A drop of water 

 placed upon it will at first be absorbed, 

 but in a short time it will be found that 



:it>> tin- tube, 

 the prcsvirr can !*• measured fan 



I the h' .1 mei 



water emerges from the cut surface, 



1 -suing from the region of the vessels. 



The -tump i- .nd to 'bleed," and may do so very extensively. 



Thus a vine ha- been observed to exude an average of 500 c.c. 



of liquid per day over .1 period of several week-. As might 



\ii< late is not pure water, but contains small quan- 

 titi' mineral and of organic matter. Occasionally the latter is 



more abundant, a- in the Sugar Maple, where the exudate from 

 incisions made into the trunk in spring, before the leaves are fully 

 formed, may contain .; per cent, of sugars. The use of apparatus 

 similar to that ill -1 in Fig. 74 -hows that a considerable pressure 



