Chapter IX — 161 — Uptake and Movement 



zontal microscope to observe the vessel contents of rooted shoots of 

 Cucurbita, Tradescantia, Impatiens, and Elatostemma. In some instances 

 the xylem could be observed directly through the transparent overlying tis- 

 sues. Removal of the outer collenchyma before observation was found not 

 to affect the state of v^^ater in the underlying xylem, if care was taken to 

 avoid jarring or destroying the ducts. The water threads remained con- 

 tinuous in spite of wilting that greatly increased the tension. Any slight 

 local disturbance was found to cause a rupture when a high state of tension 

 existed. 



Wilting and drying of leaves of sunflower and squash plants begin with 

 the most basal leaves and progresses upward. Under these conditions there 

 is found a progressively increasing number of xylem elements which contain 

 gas, either as a result of penetration from the outside through hydathodes 

 or drying areas or due to a rupture of columns caused by the strains placed 

 upon them as a result of changes in tissue form and tensions, which ac- 

 company shrivelling of the leaves (Stocking, 1943). Nevertheless, in- 

 tact water-filled elements are found even in severely wilted leaves. 



Hains (1935) combined direct observation with a dye penetration 

 method to study the state of water in the xylem of trees. By carefully peel- 

 ing off the bark and exposing the xylem he was able to detect gas in the 

 vessels, due to differences in the refraction of light by gas- and water-filled 

 elements. If no difference in appearance was noted the elements contained 

 all gas or all water. Puncturing the stem he could determine whether gas 

 or water was present and injection of dye indicated the existence of subat- 

 mospheric gas pressure or of tensions in the water columns. 



Experiments were carried out on Alnus, Acer, Aesculus, Samhiicus, and 

 Fraxinus. Gas was not present in the stems during early spring but ap- 

 peared at the onset of hot dry weather ; it disappeared during the wet fall in 

 Sambucus and Fraxinus. All outer wood of Alnus, Acer, and Aesculus con- 

 tained gas in the fall and Hains considered this to be chiefly in nonconduct- 

 ing fibres. 



From these observations, plus many that have been made on the maple 

 tree {Acer) which is noted for its bleeding in the spring, it seems that dur- 

 ing the hot, dry period of summer when water becomes deficient gas may 

 form in most of the larger xylem vessels. These may therefore serve as 

 reservoirs during the summer as well as conductors during the spring 

 when water is abundant and the flush of growth is on. During winter and 

 early spring many of these probably regain their liquid contents, but in 

 some trees gas apparently continues to occupy a fair portion of the wood for 

 it is the gas pressure formed upon warming of the trunk that causes the 

 rapid bleeding observed in maple. The original source of this bleeding sap 

 is the roots that actively absorb water as spring growth commences ; the 

 sugar results from hydrolysis of starch present in the wood parenchyma. 

 Detailed studies on this hydrostatic-pneumatic system in trees have been 

 made by MacDougal and his associates. 



Crafts (1939o?), studying Ribes inerine plants that were in a state of 

 permanent wilting, found that the bark could be removed without rupturing 

 the water columns in the outer xylem vessels. However, a slight jarring of 

 the exposed vessels, as by tapping with a dull instrument (the back of a 

 scalpel) caused formation of vapor, and recession of the water columns 

 took place so rapidly that it could not be followed ; change in appearance 

 from the translucent water-filled to the glistening gas-filled condition ap- 



