TRANSLOCATION OF SUBSTANCES IN THE PLANT 335 



force of 1,000 atmospheres. This shows the vahie of special 

 conductive tissues for dry-hind 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 only 5 cc. If these 

 values are compared with the rate at which water moves through 

 ordinary city mains, which often reaches 100 cc- per square 

 centimeter of cross section per second, or with the rate at which 

 the blood is conveyed through the arteries, normally 40 to 50 cc. 

 per second, it is found that water moves through orcUnary 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 tension developed by the 

 cells of the leaf parenchyma seems to be quite sufficient not only 

 to suspend 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, however. In the majority of deciduous trees, the vessels 

 usually do not exceed 10 cm. in length and 0.2 mm. in diameter. 

 Such smaller dimensions, though reducing conductivity, give 

 certain other advantages to the plant. The small diameter 

 diminishes the danger of breaking of the water column, while 

 the septa prevent the air, which for one reason or another may 

 have entered, from spreading through the whole system. 



Therefore it is seen that in the presence of a cohesive force, the 

 work of the upper and the lower terminal mechanisms is quite 

 sufficient to lift water to the top 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 

 elaborated sufficiently, and the resistance of the wood to filtration 

 and the role of the air entering separate vessels were considerably 

 overrated. For this reason, the idea of the insufficiency of the 



