ORGANIC SUBSTANCES AS PLANT FOOD 247 



organic nutritive substances manufactured in the leaves. On the 

 whole, this opinion remains established at the present time, with the 

 single modification that the flow of plastic substances from the 

 leaves is not always downward. It is directed not only down, into 

 the roots, but also up, to the growing tip of the stem and to the 

 developing branches and fruits. 



Moreover, the ascending flow may carry with it organic nutri- 

 tive substances as well. This usually takes place in the spring at 

 the time of sap movement, when the accumulated reserve sub- 

 stances of the wood and the bark of both the roots and the stem 

 rapidly dissolve and enter in large quantities the vessels of the 

 wood, through which they move upward to the opening buds, 

 The impelling force is the root pressure, which is especially strong 

 at this time of the year. (Arts. 43 and 44.) 



Besides the sieve tubes, the parenchyma elements of the phloem 

 also take part in the translocation of organic substances, especially 

 in herbaceous plants. Translocation through the parenchyma, 

 however, must proceed considerably slower than through the tra- 

 cheal tubes. It plays a considerable role only in translocation 

 over short distances similar to the movement of water in this 

 tissue (Arts. 60 and 61). 



The mechanism of the translocation of organic substances 

 through the plant is as yet very little understood. It is commonly 

 supposed that the conspicuous role here is played by the phe- 

 nomenon of diffusion, which forces substances to flow in the direc- 

 tion of lower concentration. In the germinating seeds, for example, 

 the greatest concentration of sugar is in the endosperm, where 

 starch is dissolved, and -the least in the embryo, where sugar is 

 used. Sugar, therefore, will flow from the endosperm into the 

 embryo. Diffusion processes, however, proceed too slowly to 

 permit one to explain the translocation of organic substances by 

 this mechanism, and still less their accumulation, as in the case of 

 soluble carbohydrates in bulbs or the roots of the sugar beet. 

 Therefore, the laws of diffusion alone are not sufficient to explain 

 the causes of the translocation of organic substances, and many 

 authors suppose that an important role is played by the move- 

 ment of the protoplasm in the conducting cells, which promotes 

 the rapid translocation of these substances. The movement of 

 protoplasm in greatly elongated cells, as for instance in the sieve 

 tubes, may be of especial importance. 



