VON MOHL'S VEGETABLE CELL. 55 



brane, developed over its surface ; and then the cell-membrane 

 (of cellulose) is deposited upon this. In Cryptogamous 

 plants, such masses, of larger or smaller size, may become 

 coated with a cell-membrane without any proper nucleus ap- 

 pearing (pp. 57-60). Thus much for what relates to the ana- 

 tomical condition of the cell. 



Under the second head, the Physiological Condition of 

 the Cell, our author treats, first, of the cell as an organ of 

 nutrition, next, as an organ of propagation, and finally, as 

 an organ of motion. He pronounces against the Knightian 

 doctrine, that plants ultimately degenerate and perish when 

 propagated for generation after generation by division (from 

 the bud) (p. 64). That the crude sap, though absorbed by 

 the parenchymatous tissue of the root, ascends through the 

 woody tissue, and that the assimilated sap returns through 

 the bark, and thence more or less into the wood by means of 

 the medullary rays, is very neatly shown. " A few simple ex- 

 periments leave no doubt about this. ... If the bark of a 

 plant, best of a tree, is cut through in a ring down to the 

 wood, there is no interruption in the flow of sap to the parts 

 situated above the wound ; but if the wood is cut through, the 

 greatest care being taken to avoid injuring the bark, that por- 

 tion of the plant above the wound dries up at once. From 

 the wood of the stem and branches the sap flows onward into 

 the leaves, as is proved by the powerful expiration of watery 

 vapor from them. Before the sap has reached the leaves it 

 is incapable of being applied to nutrition ; consequently the 

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

 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 circum- 

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

 growth of the portion below the wound stands as it were still ; 

 the stem becomes no thicker ; in the potato plant no tubers 

 are produced, etc. ; but, on the other hand, the growth above 

 the wound is increased beyond the usual measure, thicker 



