TRANSLOCATION OF SUBSTANCES IN THE PLANT 343 



^ 



the vessel, as shown at the upper part of the scheme. Tlie same 

 sugar solution in the sieve tubes absorbs water from the root cells 

 W and transfers it through the cambial cells, which retain the 

 organic substances, into the vessels of the root and stem, thus 

 creating root pressure, as shown at the lower part of the scheme. 



Evaporation of water from the cells of the leaf parenchyma 

 creates a transpiration stream, represented by simple arrows in 

 the right half of the scheme, causing a tension of water in the 

 vessels and thus contributing to the withdrawal of 

 its excess from the cells of the cambium. 



Translocation of substances in developing fruit, 

 where there is little if any assimilation and trans- 

 piration, may be illustrated by the following 

 simpler scheme (Fig. 111). The parenchyma cells 5 ^ H 



P of the leaf and stem, creating sugar or elaborat- 

 ing it from less soluble reserve substances stored 

 previously, attract water from the wood H. This 

 creates an osmotic pressure, and the flow of sugar 

 is transferred through the plasmodesmen to the 

 sieve tubes and thence to the cells of the fruit F, 

 where the monosaccharides are partly utilized in g^^^^^ o^f^^s^ 

 growth and partly stored in the form of polysac- movements in 

 charides or disaccharides. The concentration of ^'''J,''JiJ'1.enst 5". 

 the solution falls, and the water that is no longer sieve tube of 

 necessary is removed along the xylem elements, ^ykm vessel ;^,' 

 Generally speaking, in the fruit, which evaporates parenchyma cells 

 very httle water, the xylem elements serve chiefly llf\lf:^j'^fj^]^''' 

 for the outflow and not for the inflow. 



Mimch's theory, based on a series of ingenious considerations, 

 exact calculations, and very careful experiments, has given a 

 very successful solution to the question of the translocation of 

 organic substances in the plant, which has held the interest of 

 botanists from the time of the founder of plant anatomy, Mal- 

 pighi (1675). Together with the theory of cohesion elaborated 

 by Dixon (1901), with which it is very closely connected, Munch's 

 scheme gives a sufficiently complete picture of the circulation of 

 sap in the plant, somewhat analogous to the picture of circulation 

 of the blood in animals. As yet, however, it cannot be acknowl- 

 edged as completely proved, and its weakest point is the assump- 

 tion that organic substances move through plasmodesmen. 



