164 THE INDIVIDUAL ORGANISM 



perhaps sugars released from storage in stem and roots. Mineral transport 

 in the xylem seems to occur (1) more in herbaceous than in woody plants, 



(2) more in plants showing active root pressures than in those without, 



(3) more near the base of the plant than near the top, and (4) more under 

 certain special conditions of mineral abundance, root nourishment, etc. 

 than under others. The situation with regard to sugars is less clear. The 

 classical example of supposed upward movement of sugar in the xylem 

 is that of the sugar maple in spring. Experiments by Curtis showed that 

 even when sugar concentration was at a maximum in the xylem sap, and 

 when upward movement of stored sugar was probably at its peak, upward 

 transport would not take place if the phloem were completely cut. Curtis 

 thinks that although much carbohydrate is stored in the xylem, it is 

 carried there from the phloem for storage and carried out into the phloem 

 for transport up or down the stem. 



The phloem has been proved to be a two-way transport system for 

 minerals and organic substances. It carries sugars and other organic 

 materials in both directions — downward from the leaves to the stem and 

 roots where these substances are used or stored and upward from storage 

 in roots and stem. Experimental evidence is now clear that minerals 

 are also carried in the phloem in both directions; but whether the xylem 

 or the phloem is more important in mineral transport is still a matter of 

 opinion. It seems to be established that the movement of minerals into 

 leafless twigs and unexpanded leaves in spring occurs wholly through the 

 phloem and that the phloem can supply the amounts required for rapid 

 growth. This does not prove, however, that the phloem is the chief path 

 of upward transport of minerals from roots to leaves; in fact, when the 

 required amounts are small, it seems probable that either xylem or 

 phloem alone can supply them. Proteins and the simpler nitrogenous com- 

 pounds are abundant in the sieve tubes, and transport of these substances 

 seems to be one of the chief functions of the phloem. Since movement of 

 materials in the sieve tubes is by transfer through the protoplasm and 

 not by a current of water, one substance may be moving toward the roots 

 simultaneously with the leafward movement of another. 



We may pause here to contrast the vascular system of the plant with 

 the circulatory system of a higher animal. Both animal and plant possess 

 main channels for the transport of liquids to all parts of the body. There 

 the analogy ceases. In such a vertebrate as man a pumping mechanism, 

 the heart, propels through a closed circuit a fluid medium carrying oxygen, 

 food, and wastes. A circulatory system of this type may be compared 

 with an endless belt, upon which various needed materials and waste 

 products are continually deposited at different points and from which 

 these substances are continually removed for use or disposal at various 

 other points. 



