Circulation in Vegetables. 101 



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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 Lhe 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 it is 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 iu 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, I 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 overcome this power so as to raise the fluids at all? As 



