CHAPTER VII 

 TRANSLOCATION OF WATER 



69. The Path of Water Translocation. — In mosses and her- 

 baceous plants, the organs absorbing water from the soil are sep- 

 arated from those releasing it to the atmosphere. In trees the dis- 

 tance between the absorbing and the transpiring organs may be 

 several hundred feet. The water current, therefore, must move in 

 the plant through a series of tissues, beginning with the root hairs, 

 which absorb it from the soil, and ending with the cells of the leaf 

 parenchyma which set free the water vapor in the intercellular 

 spaces. 



From both a physiological and a structural point of view the 

 course of water movement in the plant may be divided into two 

 parts. Water moves mainly through the vascular system of the 

 plant, which consists of dead cells, such as the tracheids and ves- 

 sels, representing as it were water-pipe lines. In herbaceous plants 

 the distance of this path may be several centimeters, while in trees 

 water may move in this manner hundreds of feet. The second part 

 of the water-translocation system consists of living cells. Through 

 these water moves for only short distances of a few millimeters, or 

 fractions thereof. There are two small layers of tissues thus tra- 

 versed by water; one in the root, from the surface of the root hair 

 to the vessels in the central cylinder; the other in the leaf, from the 

 vessels of the fibro-vascular bundles to the mesophyll cells border- 

 ing the intercellular spaces. The course taken by the water cur- 

 rent through these tissues is represented diagrammatically in 

 Fig. 80. On the right is shown a root hair transmitting water 

 through a number of parenchyma root cells. Having passed the 

 endoderm and pericycle, it enters the vessels of the central cylinder 

 through which it is moved to the leaf. Here it passes again through 

 a number of parenchyma cells and is finally evaporated into the 

 atmosphere. 



These few millimeters of translocation in the living cells repre- 



181 



