Nutrition of Multicellular Plants - 253 



be adequate to elevate water to a height of 

 some 380 feet, although it is doubtful that 

 root pressures of this magnitude are actually 

 generated. In the root, the semipermeable 

 membrane is provided, not by the walls of 

 the ducts — since these vessels are nonliving 

 — but by the layers of living cells that inter- 

 vene between the sap in the ducts and the 

 soil water, at the external surface of the root 

 (Fig. 13-11). 



In the intact plant, transpiration, of 

 course, participates in the lifting of the sap. 

 The constant evaporation of water from the 

 leaves — and the fact that the leaves produce 

 considerable sugar — keep the chlorenchyma 

 cells distinctly hypertonic to the sap in the 

 veins. Consequently the chlorenchyma con- 

 tinually draws water from the upper parts of 

 the ducts, and this evacuation tends to lift 

 the column of sap upward in each duct. This 

 lifting power of transpiration can be ob- 

 served in a leafy branch that has been cut 

 from a plant and placed in water. In such a 

 branch, the leaves continue to transpire, and 

 can draw up enough water to maintain a 

 normal turgor in the tissues for several 

 days. 



In the springtime, before leaves have de- 

 veloped on the branches, a tree is much more 

 dependent upon root pressure for the large 

 amounts of water needed by the sprouting 

 buds. At this time the sugar content — and 

 hence the hypertonicity of the root sap — 

 reaches a maximum, because the winter 

 stores of starch in the root parenchyma are 

 converted in the spring to sugar, which 

 passes into the ducts. It may be, however, 

 that other forces, still unknown, may operate 

 to energize the upsurging of sap in the 

 springtime. 



In summer the solute content of the root 

 sap tends to drop away, because now the 

 root depends mainly upon the amount of 

 sugar that gradually comes down to the roots 

 from the leaves. Consequently, in summer 

 root pressure is relatively low, and the plant 

 depends upon the lifting power of transpira- 

 tion. 



DISTRIBUTION OF ORGANIC 

 SUBSTANCES IN PLANTS 



Part of the sugars and other organic sub- 

 stances synthesized in the leaf is utilized in 

 the metabolism of the cells of the leaf itself. 

 But a greater part passes from the leaves to 

 the lower parts of the plant, mainly via the 

 sieve tubes of phloem. 



The sieve tubes convey organic products 

 at a relatively slow rate — depending partly 

 upon the rate of passage of materials through 

 the sieve plates that interrupt the columns 

 (p. 243); partly upon the rate of streaming in 

 the sieve tube cells; and partly, perhaps, 

 upon other factors, such as electro-osmosis. 

 The rate of downward flow has been meas- 

 ured in various ways that yield equivalent 

 values of some 20 to 40 inches per hour. But 

 as the sugars and other soluble products pass 

 downward through the stem, some diffuse 

 radially, supplying the tissues at each level. 

 The rays in thicker stems facilitate this radial 

 distribution to the cells of the pith, which 

 tend to accumulate reserves of starch. The 

 remaining organic products eventually reach 

 the root parenchyma, which also stores con- 

 siderable quantities of starch, especially in 

 the case of perennial plants. 



Just before leaf fall, in hardy deciduous 

 plants, much organic content (aside from 

 cellulose) is salvaged from the leaves and 

 carried, via the sieve tube elements, to parts 

 that will survive through the winter. When 

 such transportation is completed, however, 

 the pores of the sieve plates are closed off by 

 deposition of a special high-polymer sugar, 

 called callose. Then in the following spring 

 a new set of sieve tubes, as well as a new set 

 of xylem vessels, must be formed from the 

 cambium and other meristemic tissues. 



METABOLISM IN HIGHER PLANTS 



Metabolism goes on in all the living cells 

 of the plant, producing new organic sub- 

 stances needed for the growth and mainte- 

 nance of the tissues, and providing energy, 



