194 TEXTBOOK OF PLANT PHYSIOLOGY 



sions of these cells are assumed as 0.1 mm. (they are really one- 

 third to one-half as large), it will be found that the translocation 

 of water in the parenchyma for a distance of 1 m. would require 

 the immense force of 1,000 atmospheres. This shows the value 

 of special conductive tissues for dry-land plants of large size. 

 They increase the movement of water 2,500 to 3,000 times. 



The actual translocation rate of the water through wood is not 

 high. According to the calculations of Farmer, for deciduous 

 trees, it is, on an average, 20 cc. per hour per square centimeter of 

 cross-section of wood, and for conifers but 5 cc. If these values 

 are compared with the rate at which water moves through ordinary 

 city mains, which often reach 100 cc. per square centimeter of 

 cross-section per second, or with the rate at which the blood is con- 

 veyed through the arteries, normally 40 to 50 cc. per second, it is 

 found that water moves through ordinary water pipes several 

 thousand times more rapidly than through a plant. With such slow 

 translocation of water, the resistance to filtration cannot be very 

 great. The suction pressure developed by the cells of the leaf 

 parenchyma seems to be quite sufficient not only to keep in sus- 

 pension the whole mass filling the wood, but also to draw it 

 upward. 



When the diameter of the vessels is increased and the number 

 of septa decreased, the resistance of the wood to the translocation 

 of water is considerably lowered. Such increased conductivity is 

 found in lianas, the twining and creeping plants of tropical forests. 

 The individual vessels in their stems may be 2 m. long and 1 mm. 

 in diameter. These are vessels of exceptionally large size, how- 

 ever. In the majority of deciduous trees the vessels usually do not 

 exceed 10 cm. in length and 0.5 mm. in diameter. Such smaller 

 dimensions though reducing conductivity, secure certain other 

 advantages to the plant. The small diameter diminishes the dan- 

 ger of breaking of the water threads, while the septa prevent the 

 air, which for one reason or another may have entered, from spread- 

 ing through the whole system. 



Thus it is seen that in the presence of a cohesion force, the 

 work of the upper and the lower terminal mechanisms is quite suf- 

 ficient to lift water to the crown of a tree. The main containers of 

 the water stream remain as passive as the iron pipes of a water- 

 system. Formerly, however, the cohesion theory was not elab- 

 orated sufficiently, and the resistance of the wood to filtration and 



