TRANSLOCATION OF WATER 191 



the vascular bundles, which pervade the whole leaf. Consequently, 

 a pull on the water in these vessels is produced, which is trans- 

 mitted to the vessels of the stem and the roots. The water in the 

 wood appears to be suspended to the cells of the leaf parenchyma, 

 but in the root tips are also parenchyma cells which, with a suffi- 

 cient amount of water in the soil and with a slow utilization, sup- 

 port, as it were, the water threads suspended from the leaf cells. 

 Thus the strain on the leaf parenchyma is reduced and the water 

 threads are even pushed upward with considerable force. With 

 lack of water in the soil and with a simultaneous intensive trans- 

 piration, as, for instance, on hot summer days, the supply of water 

 by the root cells cannot keep up with the loss of water from the 

 leaves; in this case the influence of suction of the leaves is trans- 

 mitted through the highly strained threads in the vessels, even as 

 far as the root cells. 



63. Compression of Vessels during the Ascent of Water. 

 The Presence of Air in Vessels. — The water threads filling the 

 vessels tend to increase in length and to decrease in diameter, sim- 

 ilar to a stretched rubber tube. This tendency would soon cause 

 their breaking into separate drops, which is actually observed when 

 water falls in a thin jet, if they were not contained within the 

 walls of vessels to which they are closely attached by the force of 

 cohesion. When water is under a very high strain in the vessels, 

 the walls of the vessels will be drawn inward and the diameter will 

 decrease. This transverse compression of the vessels is counter- 

 acted by the annular or spiral thickenings, which keep the walls 

 apart. On the whole, the compression is rather small. It may be 

 perceived under the microscope by observing the vessels of plants 

 that have a sufficiently transparent stem, for instance young plants 

 of pumpkin or balsam, during rapid wilting, when the suction ten- 

 sion of the leaves attains its maximum, while the roots are unable 

 to supply water from the dry soil. This compression of vessels 

 may be observed also on a whole tree trunk by means of the den- 

 drograph. This is an extremely sensitive apparatus, devised by 

 MacDougal, which records slight changes in the thickness of the 

 trunk (Fig. 87). This apparatus shows that during the afternoon 

 hours of bright days, a contraction of the trunk takes place, fol- 

 lowed by an expansion at night (Fig. 88). 



The elastic compression of the walls of vessels during periods 

 of increased transpiration is the cause of a phenomenon which has 



