THE MOVEMENTS OF FLUIDS IN STEMS. 207 



diminished tension. The following are some of Hoehnel's 

 results : 



Quercus pedunculata, 

 .(Esculus hippocastanum, 

 Syringa vulgaris, 

 Ulmus campestris, . 

 Helianthus, 



24-5 cm. 

 37" cm. 

 24" cm. 

 20- cm. 

 46* cm. 



These experiments show that the mercury will rise in a few 

 seconds from 20 to 46 centimetres, from the diminished tension 

 of the air in the vessels. 



The water passing up through the xylern goes to the leaves, and 

 is there transpired as watery vapour through the stomata, at 

 least in great part. The conditions necessary for transpiration 

 are light and heat, the light having a powerful influence in open- 

 ing the stomata of the leaf and promoting transpiration. The 

 rapidity of the upward current in the xylem is very great, the 

 astonishing rate of 13 inches per minute having been observed 

 by Pfitzer in the stem of Helianthus annuus. In other plants, 

 transpiring more slowly, as in the cherry laurel, the fluid passes 

 through the xylem at the speed (as observed by experiment) of 

 40 inches per hour, when the external conditions are favourable. 

 The rapid current is a current of water running from the roots to 

 the leaves through the walls of the cells of the xylem, not in the 

 vessels or cavities of the cells. It is a variable current, begins 

 when the leaves begin to unfold, and ceases when the leaves cease 

 to be active a little before their fall. It also varies in accordance 

 with the alternation of day and night, being active during the 

 day, but slow or almost ceasing during the night. Then during 

 the day the speed varies in accordance with the degrees of heat 

 and light, increasing when the temperature and light are greatest, 

 and slowing as they diminish. 



In addition to the rapid current there is the slow current, a 

 constant or nearly constant one, uninfluenced by external condi- 

 tions, such as alternation of light and darkness, or variations in 

 temperature and light. The determining cause of this current is 

 osmose, the fluid moving to those cells that have the protoplasm 

 dense, and thus require water, a fact clearly seen in the young 

 parts of a shoot which retain their freshness, while the older parts 

 wither and dry up. This slow movement is in the cavities of the 

 parenchymatous cells, and conveys all the nutrient matters to and 

 from growing and assimilating parts; and during their convey- 



