470 BOTANY 



plant must absorb from the soil the amount necessary to restore the 

 loss caused by evaporation, as well as the water needed to convey the 

 food constituents. 



The factors concerned in the movements of water within the plant- 

 body are extremely complicated, and are still not entirely under- 

 stood. In plants composed of simple cellular tissue, like most Algae, 

 the movements are due, probably, entirely to osmotic agencies ; but 

 in large complicated plants, like trees, where the water must all be 

 absorbed from the soil, such cell-to-cell transfer is not sufficiently 

 rapid to provide for the transmission from the roots to the actively 

 transpiring leaves at the summit. Hence in such plants special con- 

 ducting tissues are developed the vascular bundles. It is the 

 xylem, or woody part of the vascular bundles, which is preeminently 

 the water-conducting tissue (Hadrom). 



The water osmotically absorbed by the root-hairs passes into the 

 vascular bundle of the root, whence it is conveyed, mainly through 

 the agency of the tracheary elements, and probably partly by capil- 

 larity, to the tracheary tissue of the stem-bundles, and thence to the 

 transpiring leaves. The loss of water in the latter, due to evapora- 

 tion, is, of course, an important factor in regulating the upward 

 current of water from the roots. 



The evaporation from the aerial parts of a plant in clear, hot 

 weather is very great, and the disturbance of the equilibrium thus 

 caused must powerfully affect the movement of water in the stem. 

 But just how far the upward movement is a purely mechanical one 

 through the dead tracheary tissue, and to what extent it is influenced 

 by the living cells adjacent, is still undetermined. 



The tracheary tissue is not all equally active in the transport of 

 water. In large woody stems, such as the trunks of trees in which 

 " heart-wood " is present, it is only the outer wood which is active. 

 The heart-wood is almost entirely without importance in the conduc- 

 tion of water. 



That the vascular bundles are the principal channels of conduction 

 for solutions can be shown by a simple experiment. If the cut end 

 of a flower-stalk of a white flower, like a Narcissus or Lily, is placed 

 in a colored solution, such as a watery solution of indigo, carmine, 

 eosin, etc., the dye is carried rapidly up the stalk, and follows the 

 delicate veins in the white floral leaves, in which the veins are 

 clearly traced by the colored fluid. In species of Cucurbita, the 

 ascent of the fluid may be at a rate of six metres in an hour. 



Transpiration 



The amount of water lost by transpiration varies with the tem- 

 perature and air-moisture. While a small amount of water may be 



