TRANSLOCATION OF SUBSTANCES IN THE PLANT 323 



which consists of dead cells, such as tracheids and vessels, repre- 

 senting as it were water-pipe Unes. In herbaceous plants, the 

 length 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 less. There are two small layers of tissues thus 

 traversed 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 fibrovascular bundles to the mesophyll 

 cells bordering the intercellular spaces. The course taken by 

 the water current through these tissues is represented diagram- 



FiG. 102. — Diagram showing the course taken by water current in the plant. 

 The black arrows show movement of water in liquid form; the light arrows, that 

 of water vapor {after Smith, et al.). 



matically in Fig. 102. On the right is shown a root hair trans- 

 mitting water through a number of parenchyma root cells. 

 Having passed to 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 milHmeters of translocation in the living cells are 

 subject to greater difficulties than are^the many meters traversed 

 in the conductive tissues. The ease with which water may be 

 moved through the vessels and its slow movement from cell to 

 cell may be seen from the following simple experiment. When 

 a succulent leafy stem with numerous parenchjona cells, for 

 instance, the stem of the balsam plant, is cut and placed in 

 water, it will remain fresh for a long time, as the water readily 

 enters through the cut vessels and ascends to the leaves. When 

 the cavities of the vessels are sealed up by immersing the cut 



