MOVEMENT OF MATERIALS IN THE PLANT 



133 



metal. Occasionally vessels may thus be injected with mercury for a distance 

 of from 50 to 60 cm. above the cut surface. Experiments of this kind show that 

 the attenuation of the air in the vessels may be very considerable. Negative 

 pressure in wood may be demonstrated in still another way. A leafy branch 

 with two or more twigs (Fig. 77) is placed with its cut end in water. One of 

 the twigs is cut off and the cut end (b) is connected with rubber tubing to a glass 

 tube (a), the lower end of which dips into mercury. After some time the mer- 

 cury rises in the tube indicating that the air in the wood is rarefied. The air 

 of the stem is most attenuated when the activity of the plant is greatest. As 

 will be seen later, this phenomenon of negative gas pressure bears an important 

 relation to the movement of water in the stem. 



Fig. 76. — The cutting of a stem under mercury 



§3. Movement of Water and Dissolved Substances.— The first experiments 

 upon the movement of water and solutes in plants were carried out by Malpighi 

 in 1 67 1. He removed a ring of bark from a woody stem and found that the 

 region above the wound continued alive and grew even more rapidly than be- 

 fore, producing an annular swelling (Fig. 78), while the region below the wound 

 failed to develop further. The girdling operation is thus seen to have no effect 

 at all upon the movement of water from the soil into the upper portion of the 

 plant, although it stops the movement of organic materials into the lower 

 regions. Malpighi concluded from this experiment that the soil solution moves 

 upward through the wood, while the organic substances produced in the leaves 

 pass downward through the cortex. The movement of water is sometimes 

 spoken of as the ascending current, and that of organic (or plastic) substances 

 as the descending current. The expressions ascending and descending are not 

 to be interpreted literally, however; in the drooping branches of the weeping 

 willow, for example, the ascending stream descends and the descending one 



c The phenomenon is mainly dependent upon the rate of loss of water by transpiration 

 from the leaves and upon the rate at which water reaches the leaves from below. The word 

 activity, as used in the text, is rather indefinite, but it may be taken to refer to conditions 

 promoting high transpiration rates. — Ed. 



