MOVEMENTS OF WtTTEIEHT MATTEES. 343 



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 demon- 

 strate, indeed, that they do go down in the bark. We 

 have, however, 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 evaporation, 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 rapid- 

 ly 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. wHcb the matters they conduct are organized. 



In endogenous plants and in some exogens (Piper me- 

 dium, Am,aranthus sanguineus) the vascular bundles con- 

 taining sieve-cells pass into the pith and are not confined 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, (Hanstein,) 

 and not above the girdled place. 



In all cases, without exception, the matters organized in 

 the leaves, though most readily and abundantly moving 

 downwards in the vascular tissues, are not confined to 

 them exclusively. When a ring of bark is removed from 

 a tree, the new cell-tissues, as well as the vascular, are in- 

 terrupted. Notwithstanding, matters are transmitted 

 downwards, through the older wood. When but a narrow 

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