THE VEGETABLE CELL. 71 



different layers, without a movement of the Rap ; but we are 

 wholly ignorant of all that relates to this. The case is differeDt 

 with the Phanerogamiaj in which the different processes connected 

 with the nutrition are committed to different organs ; here we at 

 least know somewhat more accurately the course which the sap 

 describes, at all events in the Dicotyledons 



A few simple experiments leave no doubt about this. The 

 watery fluids are, as we have seen, absorbed by the cells lying at 

 the surface of the rind of the root, they flow no further, however, 

 in the rind, but pass into the wood, even in the small roots, and 

 ascend in this through the stem and branches. The proof of this 

 is fm^nished by two facts : if the bark of a plant, best of a tree, 

 is cut tln'ough in a ring down to the wood, there is no interrup- 

 tion of the flow of sap to parts situated above the wound ; but if 

 the wood is cut through, the greatest care being taken to avoid 

 injuring the bark, that portion of the plant above the wound 

 dries up at once. From the wood of the stem and branches the 

 sap flows onwards into the leaves, and in these into their paren- 

 chymatous tissues, as is proved by the powerful expiration of 

 Water vapour from them. Before the sap has reached the leaves 

 it is incapable of being applied to the nutrition ; conseq^uently, 

 the vegetation of a plant comes to a stand still when it is de- 

 prived of its leaves. The sap ascending from the root to the 

 leaves is thence termed the crude sap. It undergoes a chemical 

 change in the leaves, rendering it fit to be applied to the nutrition 

 of the plant. To this end the sap flows backwards from the 

 leaves through the bark, to the lower parts, as the following cir- 

 cumstances testify. If the bark is cut off the stem in a ring, 

 the grbwth of that portion of the plant below the wound stands 

 as it were still, the stem becomes no thicker, in the Potato plant 

 no tubers are produced, &c. ; but on the other hand, the growth 

 above the wound is increased beyond the usual measure, very 

 tliick layers of wood are deposited, more fruit is perfected, these 

 ripen sooner, &a The deposition of starch which occurs in the 

 cells of the medallary rays in Autumn, goes to prove that the 

 portion of assimilated sap which is not used for nutrition on the 

 way to the root, runs back to the wood through these horizontal 

 medullary rays, and thxis the sap describes a kind of circle, not, 

 indeed, in determinate vessels, but in a definite path leading 

 through the different parts of the plant. 



Observ, It is difficult to conceive how in recent times the results of 

 these experiments (for the details of which reference should he made espe- 

 cially to Buhamefs ^^ Physique des arhres'^ and Cottars ^^ N aturheohmh- 

 twhgen uh. d. Bewegung des Sa/ies") could have been questioned, and the 

 existence of the descendmg current of sap in the hark denied. Certainly 

 it is no improvement on the theory cast aside, when the increased growth 

 above the annular wound is explained by artificial interruption of the 

 upward current of crude sap, in consequence of which the sap coutaiaecj 



