NUTRITION or VEGETABLES. 147 



on the outside. The cells of the green parts of the bark and leaves fill with chloro- 

 phyll. The latex of the laticiferous vessels becomes charged with coloured granules, 

 and the sap, thickened and enriched with new principles, descends from the leaves 

 along the inner surface of the bark towards the roots. This descending movement 

 is easily proved ; it is sufficient to prune the bark of a young branch to see the sap, 

 if it is coloured, ooze from the upper lip of the incision and not from the lower. If 

 the stem be tightly corded, after some time the bark swells, and forms a cushion 

 above the ligament, whUe the stem below will preserve its original diameter. For 

 this reason the elaborated sap is also called descending sap. 



The elaborated sap furnishes the cambium, a gelatinous fluid which permeates 

 the cellular zone, and in yvhich are formed the elementary organs which combine to 

 produce growth in the vegetable. 



In dicotyledonous stems, the cambium is principally deposited between the 

 woody and cortical systems, within the layer of laticiferous vessels and the fibres 

 of the liber, in contact with which the descending sap flows. The young buds 

 springing from the axil of a leaf are placed in the direction of the flow of latex 

 from that leaf, and which, accumulating at the base of the petiole, elaborates there 

 the elements of cambium. 



In monocotyledonous stems, the fibres analogous to the liber and the vessels of 

 the latex, which each fibro-vascular bundle contains, furnish an elaborated sap, which 

 deposits cambium in heaps dispersed through the stem ; so that their terminal bud 

 profits by the sap elaborated by the leaves of the preceding bud. 



Finally, rain, containing the materials for the food of the vegetable, is absorbed 

 by the tips of the roots, rises in the stem, crosses the wood system, reaches the 

 parenchyma of the leaves and the cellular tissue of the barb, where it undergoes the 

 action of the air, becomes elaborated sap, descends through the bark, deposits a 

 zone of cambium between the liber and alburnum, and arrives at the tips of the 

 roots, whence it started ; thus establishing a true circulation. 



Oyclosis is a peculiar circulation which Schultz has discovered in the laticiferous 

 vessels ; he observed that the coloured granules flow in sinuous tracks, being carried 

 by the latex currents in various directions along the courses of the anastomosing 

 laticiferous vessels. 



Physiologists have proposed different theories to account for the propelling 

 force which puts the latex in motion ; but Mohl has shown that this motion is not a 

 vital phenomenon, but that it always arises, either from a rent in the tissue, whence 

 the latex necessarily escapes, or from a mechanical pressure on the tissue, which 

 sets the latex in motion; as also that this motion soon ceases. 



But if cyclosis is an obscure and doubtful phenomenon, this is not the case with 

 the intercellular circulation {rotation), which can be observed in the septate hairs of 

 certain plants (Tradescantia), and especially in the cells of certain aquatics (as 

 Cha/ra) . Chara is a leafless, frondless acotyledon ; its internodes, whether isolated 

 or in bundles, consist of cylindrical cells placed end to end ; each internode pro- 

 duces at its top a whorl of cells similar to itself, which speedily become similarly 

 septate. If one of these cells be placed under a microscope, and cleared from the 



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