Circulation in Vegetables. 101 
in them, in order that they may be fitted for the nutriment of the 
vegetable. 
1. It is evident that changes must take place in the fluids absorb- 
ed by the roots, to fit them for the nutriment of the plant. These 
changes commence, as soon as they are taken into the vegetable sys- 
tem: the most important change however is effected as they pass 
upward through the neck of the vegetable. This view of the sub- 
ject is supported by the fact that the neck is the most important part 
of the plant. It is a vital part; divide a plant here and itis destroyed. 
The structure of this part appears different. But what seems to fa- 
vor, still more, the opinion expressed above, is the change, which 
the fluids visibly undergo in passing through this part. ‘To be con- 
vinced of this the reader may examine the Asclepias Syriaca, or 
milk weed. Divide the root below the neck, and the fluid which ex- 
udes is merely watery, but when the division is made at the neck, 
that peculiar milky fluid appears. In this case, we must admit that 
the fluid is changed as it passes upward, or as Mr. Knight supposes 
in the leaves and then passes down to the neck and there stops. If 
the latter supposition be true, would not accumulations of sap take 
place in this part. But this is not the case. 2. No power has been 
pointed out which can cause a descent of the sap in the uninjured 
vegetable, while the roots remain in the earth. It appears to me 
that it is unphilosophical to maintain that gravity is a principal cause 
of the descent of the cambium, as Mr. Knight has stated in the 
following passages in the Phil. Trans. for 1803. . “These causes, 
(that is of the descent of the sap,) appear to be gravitation, motion 
communicated by the winds or other agents, capillary attraction, and 
probably something in the conformation of the vessels themselves 
which renders them capable of carrying the fluids in one direction 
rather than another. pp. 277-8. Again, when a tree is deprived of 
all motion by being trained to a wall, or when a large tree has been 
deprived of its branches, it becomes unhealthy and not unfrequently 
perishes, apparently from a stagnation of the descending fluids under 
the rigid cincture of the lifeless external bark. p. 282. Another 
cause of the descent of the sap to the root, 1 have supposed to be 
capillary attraction and something in the conformation of the vessels 
themselves ; I however consider gravitation as the most extensive 
and active cause of motion of the descending fluids of trees. p. 
283.” But if gravitation be the active cause, &c., how can the 
vegetable cvercome this power so as to raise the fluids at all? As 
