DIFFUSION OF FLUIDS. 573 



stem that is, in the cambium-layer of the wood and in the in- 

 ternal tissue of the bark. This supplies the material for the de- 

 velopment of new wood in the fibro- vascular layers ; and this elabo- 

 rated sap evidently passes not only downward through the latticed 

 vessels, sieve tubes, vasa propria, and conducting cells, but inward, 

 by lateral transmission (since we find in autumn starch-granules 

 laid up in the medullary rays between the wedges of developed 

 wood), and also upwards when growth is going on, or where reserve 

 material has to be accumulated. 



The sap therefore is transferred from place to place in varying 

 directions in Dicotyledons, not in a proper system of vessels, but 

 by a series of disturbances and restorations of equilibrium in a mass 

 of permeable tissues. The changes are dependent upon local phy- 

 sical, chemical, and developmental actions. 



The evidence of a descent of elaborated sap is overwhelming. The 

 simplest proof, that of removing a ring of bark, which causes the arrest 

 of development of wood below the ring, is borne out by all variations 

 of it. Ringing fruit-trees in this way causes a swelling of the tissues 

 and a temporary increase of product of fruit above or on the distal 

 side of the wound, from the accumulation of the elaborated matter. 

 The formation of tubers in the Potato and similar plants is prevented 

 by interrupting the continuity of the cortical layers; and when hark 

 is removed in patches, and the surface becomes gradually grown over 

 by new wood, the greater part of the new growth comes from the 

 upper side. Still a descending current is not the only direction in which 

 nutritive fluids flow ; for, as has been already stated, the flow mav be in 

 any direction, and new wood may be formed in place without immediate 

 connexion with any descending current. 



It is Mulder's view that all the nitrogenous constituents of plants are 

 not only absorbed by the roots, but assimilated there at once, and that 

 carbon is fixed in the green organs then, that a continual interchange 

 goes on from above and below, the roots supplying protoplasmic matters 

 which originate all organic phenomena, while the leaves send down the 

 ternary compounds (C H O) which afford the material for cell-membrane, 

 starch^ &c. This author attributes the distribution to simple endosmose ; 

 hut this does not account for the passage of crude sap through the albur- 

 num, and of elaborated nutriment through the inner bark. Other authors 

 consider that organic substances (carbo-hydrates, albuminoids, &c.) are 

 formed in the leaves ; in such a case a descent of the sap must of neces- 

 sity occur. The transfer from one leaf to another of such substances as 

 glucose, albumen, phosphates, &c. may be accounted for by evaporation. 



Sachs states that the elaborated sap in the cellular tissue is different from 

 that in the vascular ; " the parenchymatous tissues have," says he, " an 

 acid sap, containing sugar, starch, oil, vegetable acids," &c. The vascular 

 and prosenchymatous tissues, including the " vasa propria " and clathrate 

 cells and other elements of the soft bast, have an alkaline sap. The sap 

 passing through these tissues is of an albuminous or nitrogenous nature. 

 Other physiologists, however, doubt whether any such sharply defined dual 

 nature of the elaborated sap exists, though admitting the large share 



