How Substances are Transported and Removed 213 



in a copious and continuous current through the plant. This 

 involves of course a highly efficient water-carrying mechanism, 

 which we should first consider. The principal feature thereof is 

 the ducts, which are tubes, beginning near the tips of the roots 

 (figure 53) and running in bundles throughout the length of the 

 stem to the leaves, as our earlier generalization of the system 

 so clearly illustrates (figure 54, A) ; and here they end in little areas 

 of green tissue, as we have noted already in the description of the 

 leaf. Structurally, the individual ducts are short, but the end of / 

 each one lies against the end of another with only a thin partition / 

 between; and therefore the practical effect is that of a continuous I 

 tube with occasional thin cross partitions. When roots and stems ^ 

 are young and flexible, the soft walls of the ducts are supported 

 inside by ringed or spiral thickenings, which keep the cavities 

 open when the young roots or stems become sharply bent back 

 by accident, and also against the turgescent pressure of neighbor-, 

 ing cells. The ducts formed later, however, when the tissues are 

 thicker and harder, have not the spirals, but stiff bands or a fret 

 work, or even a uniform thickening, pierced by thin areas for 

 the escape of some water to the neighboring tissues. These dis- 

 tinctive features of ducts are very well shown in the picture 

 given herewith (figure 72; also 54, C). 



We turn now to the study of the transfer of water through the 

 plant, or, as it may also be expressed, the forces impelling the 

 ascent of sap. Transpiration makes very great demands for a 

 water supply, especially in lofty and broad-leaved trees, and in 

 weather that is bright, dry, and windy. By what forces is so 

 weighty a volume of water raised so quickly to a height so great? 

 Recently I had occasion to calculate the work done in a day in 

 transferring the water from roots to leaves in one of the largest 

 kind of trees, and I found it was just about equal to that which 

 would be done by a man in carrying 500 large pailfuls of water 

 up a ten-foot flight of stairs within ten hours. This is nearly a 

 pailful a minute for ten hours without cessation, my figures being 



