384 HOW CROPS GROW. 



cell-tissue in order to reach the point where they nourish 

 the growing organs. 



Evidence that nutrient matters also pass upwards in 

 the bark is furnished, not only by tracing the course of 

 colored liquids in the stem, but also by the fact that 

 undeveloped buds perish in most cases when the stem is 

 girdled between them and active leaves. In the excep- 

 tions to this rule, the vascular bundles penetrate the 

 pith, and thereby demonstrate that they are the chan- 

 nels of this movement. A minority of these exceptions 

 again makes evident that the sieve-cells are the path of 

 transfer, for, as Hanstein has shown, in certain plants 

 (SolanacesB, Asclepiadeae, etc.), sieve-cells penetrate the 

 pith unaccompanied by any other elements of the vascu- 

 lar bundle, and girdled twigs of these plants grow above 

 as well as beneath the wound, although all leaves above 

 the girdled place be cut off, so that the nutriment of the 

 buds must come from below the incision. 



The substances which are organized in the foliage of a 

 plant, as well as those which are imbibed by the roots, 

 move to any point where they can supply a want. Garb- 

 hydrates pass from the leaves, not only downwards, to 

 nourish new roots, but upwards, to feed the buds, flow- 

 ers, and fruit. In case of cereals, the power of the 

 leaves to gather and organize atmospheric food nearly or 

 altogether ceases as they approach maturity. The seed 

 grows at the expense of matters previously stored in the 

 foliage and stems (p. 237), to such an extent that it may 

 ripen quite perfectly although the plant be cut when the 

 kernel is in the milk, or even earlier, while the juice of 

 the seeds is still watery and before starch-grains have 

 begun to form. 



In biennial root-crops, the root is the focus of motion 

 for the matters organized by growth during the first 

 year ; but in the second year the stores of the root are 

 completely exhausted for the support of flowers and seed, 



