SECRETED FLUIDS. 65 



rish the plant, and our latest inquiries confirm the 

 suggestion. Du Hamel compares them to the blood 

 of animals, and so does Darwin. But the analogy 

 seems more plain between the sap, being nearly uni- 

 form in all plants, and the animal blood ; as in that 

 particular they accord, while the secreted fluids are so 

 very various. Mr. Knight's theory confirms this ana- 

 logy, at the same time that it establishes the opinion of 

 Malpighi. The sap returning from the leaf, where it 

 lias been acted upon by the air and light, forming new 

 wood, is clearly the cause of the increase of the vege- 

 table body. But it is not so clear how the resinous, 

 gummy or other secretions, laid aside, as it were, in 

 vessels, out of the great line of circulation, dan direct- 

 ly minister to the growth of the tree. I conceive they 

 may be in this respect analogous to animal fat, a reser- 

 voir of nourishment whenever its ordinary supplies are 

 interrupted, as in the winter, or in seasons of great 

 drought, or of unusual cold. In such circumstances 

 the mucilaginous or saccharine secretions especially, 

 perhaps the most general of all, may be absorbed into 

 the vegetable constitution ; just as fat is into the ani- 

 mal one, during the existence of any disease that inter- 

 rupts the ordinary supplies of food, or interferes with 

 its due appropriation. It is well known that such 

 animals as sleep through the winter, grow fat in the 

 autumn and awake very lean in the spring. Perhaps 

 the more recent layers of wood in a Plum- or Cherry- 

 tree,, if they could be accurately examined, might be 



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