72 ANATOMY AND PHYSIOLOGY OF 



in the upper part of tlie plant, must soon become greatly concentrated 

 and potential for development (Sclileiden, " Gruiich.'' 2iii(l ed. II. 513). 

 Wlien we can succeed in fattening an animal by depriving it of a portion 

 of its accubtomed food, t3ii& explanation may be received as satisfactory. 



Mulder bIbo {" Fh^siolog, Ohem.'") denies tliat iliere exists a downward 

 current of tlie sap, aitbougli lie does not call in question the fact that the 

 nutrient matters formed in the leaves do descend. That is to say, he 

 assumes that the substances which the sap carries upwards are exchanged, 

 according to the laws of endosmobo acting in the ascending sap, with 

 those substances which are elaborated in the leaves. If this were the case, 

 the latter nutrient matters must descend in the same coiu'se and through 

 the very cells in which the sap ascends, i e., through the wood; the above- 

 mentioned experiments demonstrate that they certainly do not, but 

 remain in the upper parts of the plant when this path is freely open to 

 them. 



The diiferont layers of the wood do not convey the sap in ^qnal 

 quantity ; the outermost, youngest layers, and in stems not more 

 than two years old also the mea"allary sheath, principally preside 

 over the conveyance of the sap. The older a tree becomes, and 

 the harder the wood it possesses, the less share do the older layers 

 take in the conveyance of sap ; hence, trees with hard wood, like 

 the Oak, where the sap wood exists in a circle round the stem, dry 

 rapidly; while in trees with soft wood, like the Bii'ch, the central 

 layers of wood still carry sap, even in thick trunks. 



When the question arises as to which elementary organs the 

 sap ascends in, and by what force it is lifted upwards, we arrive 

 at a region wherein a]i is still obscure, but in which so many the 

 more hypotheses have been ventured. 



In the first place, two views stand diametrically opposed to each 

 other ; according to one, the conveyance of the sap is committed to 

 the vessels ; according to the other, these ca^ry air, and the sap 

 flows in the cellular tissue. The adherents of the first opinion (to 

 which belonged Malpighi, Duhamel, Treviranus, Link) chiefly de- 

 pended upon the circumstance that when cut plants were placed 

 in coloured fluids, these became diffused through all parts of the 

 vascular system, a conclusion which, while referring to processes 

 occurring in healthy plants, takes its stand on plants placed 

 in most unnatural circumstances, and is now not considered 

 valid by any one. In like manner, no great weight can be laid 

 upon the phenomenon of the sap flowing from the cut vessels when 

 trees such as the Birch, Maple, Vine, &c., are wounded in Spring; 

 since these plants are in such diflerent conditions before the un- 

 folding of their leaves and in later periods of their vegetation, that 

 a conclusion from one to the other must be regarded as inadmis- 

 sible. More important to the theory of the conveyance of the sap 

 are the experiments of Link Q^Ann, de sc, natnr, XXIII f' 144 — - 

 " Varies m. KrauPTku^idc" i, 116), according to which, plants 

 which have been watered for some days with a solution of ferro 



