MOVEMENTS OF ^UTIUENT MATTEES. 345 



thereby demonstrate that they are the channels 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 (Solan acea3, Asclep- 

 iacleae, etc.,) sieve-cells penetrate the pith unaccompanied 

 by any other elements of the vascular 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 be- 

 low 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. Car- 

 bohydrates pass from the leaves, not only downwards, to 

 nourish new roots, but upwards, to feed the buds, flowers, 

 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. 218,) 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 com- 

 pletely exhausted for the support of flowers and seed, so 

 that the direction of the movement of these organized 

 matters is reversed. In both years the motion of water is 

 always the same, viz., from the soil upwards to the leaves.* 



The summing up of the whole matter is that the nutri- 



* The motion of water is always upwards because the soil always contains 

 more water than the air. If a plant were so situated that its roots should 

 steadily lack water while its foliage had an excess of this liquid, it cannot, he 

 doubted that then the " sap " would pass down in a regular flow. In this case, 

 nevertheless, the nutrient matters would take their normal course. 



15* 



