382 HOW CROPa GBOW. 



The first maifestation of growth is the formation of a' protuberance at 

 the lower edge of the bark, which is known to gardeners as a callous, 

 C. This is an extension of the cellular tissue. From the callous shortly 

 appear rootlets, B, which originate from the vascular tissue. Rootlets 

 also break from the stem above the callous and also above the water, 

 if the air be moist. They appear, likewise, though in less number, 

 below the girdled place. 



Nearly all the organic substances (carbhydrates, al- 

 buminoids, acids, etc.) that are formed in a plant are 

 produced in the leaves, and must necessarily find their 

 way down to nourish the stem and roots. The facts 

 just mentioned demonstrate, indeed, that they do go 

 down in the bark. We have, howeyer, no proof that 

 there is a downward flow of sap. Such a flow is not 

 indicated by a single fact, for, as we have before seen, 

 the only current of water in the uninjured plant is the 

 upward one which results from root-action and evapora- 

 tion, and that is variable and mainly independent of the 

 distribution of nutritive matters. Closer investigation 

 has shown that the most abundant downward movement 

 of the nutrient matters generated in the leaves proceeds 

 in the thin-walled sieve-cells of the cambium, which, in 

 exogens, is young tissue common to the outer wood and the 

 inner bark — which, in fact, unites bark and wood. The 

 tissues of the leaves communicate directly with, and are 

 a continuation of, the cambium, and hence matters 

 formed by the leaves must move most rapidly in the 

 cambium. If they pass with greatest freedom through 

 the sieve-cells, the fact is simply demonstration that the 

 latter communicate most directly with those parts of the 

 leaf in which the matters they conduct are organized. 



In endogenous plants and in some exogens {Piper me- 

 dium, Amaranthus sanguineus), the vascular bundles 

 containing sieve-cells pass into the pith and are not con- 

 fined to the exterior of the stem. Girdling such plants 

 does not give the result above described. With them, 

 roots are formed chiefly or entirely at the base of the 

 cutting (Hansteih), and not above the girdled place. 



