DESCENDING SAP. 55 



OF THE USE AND MODE OF DISTRIBUTION OF THE 



NUTRITIVE JUICES. 



* 



45. The sap elaborated in the leaves, as we have seen, again 

 descends to other parts of the plant, and constitutes the nutritive 

 juice by the aid of which its growth is effected. 



46. It is easy to be convinced that the nutritive juices of plants 

 are formed in the leaves ; for if we strip a tree of all its leaves, 

 it will cease to grow until it is furnished anew with these organs ; 

 and farmers who cultivate mulberries for feeding silkworms have 

 remarked that the growth of the trees is less in proportion to the 

 frequency of stripping them of their leaves. 



47. The movement of the nutritive juice (that is, the descend- 

 ing sap) is slow, and always takes place from the leaves, towards 

 the roots, whatever may be the position of the branches that this 

 liquid traverses. 



48. The route followed by the descending sap is not the same 

 as that by which the sap rises from the roots to the leaves ; in- 

 stead of traversing the ligneous layers, it descends chiefly through 

 the substance of the bark. 



49. The following experiment proves that it is the descending 

 sap which especially serves for the nutrition of the plant, and that 

 this same sap moves in the interior of the bark. If we remove 

 from a branch or the trunk of an ex'ogenous tree, a circular strip 

 of bark, we prevent the sap that descends from the leaves to the 

 lower part of the plant from continuing its route, and, in fact, we 

 see that the portion of the stem which is below this annular or 

 ring-like section, ceases to grow, while the part situate above 

 profits more than is usual, and swells out on the upper margin 

 of the wound, so as to form a ring. The same thing happens 

 when we surround a branch by a very tightly drawn cord ; for 

 in this way we may also arrest the descending sap, and the parts 

 where this juice accumulates are benefited at the expense of those 

 situated below. 



45. What becomes of the sap that is elaborated in the leaves ? (Elaborate: 

 from the Latin, labora're, to work. The word is employed to signify the 

 act of living organs upon substances capable of assimilation, by which 

 nutritive matter is separated and appropriated. The elaboration of food in 

 the stomach produces chyme.} 



46. What proof is there that the nutritive juice of plants is formed in 

 the leaves ? 



47. Is the movement of the nutritive juice rapid ? In what direction 

 does it flow ? 



48. What is the route of the descending sap ? 



49. How do you prove that the descending sap is the nutritive juicu o' 

 plants, and that it moves through the substance of the bark? 



