SECRETED FLUIDS, 81 



could be accurately examined, might be found to con- 

 tain a greater proportion of mucilage at the end of au- 

 tumn than in the early spring. If these substances do 

 not nourish the plant, they seem to be of no use to it, 

 whatever secondary purposes they may answer in the 

 schemes of Providence. The direct end, with respect 

 to the plant, of the finer secreted fluids of its fruit can 

 very well be perceived, as tempting the appetite of an- 

 imals, and occasioning, through their means, the disper- 

 sion of the seeds ; and the perfume of flowers may at- 

 tract insects, and so promote the fertilization of the seed, 

 as will be explained hereafter. 



After what has been said, we need not waste much 

 time in considering the hypothesis, advanced by some 

 philosophers, that the sap- vessels are veins and the re- 

 turning vessels arteries. This is so far correct, that, as 

 the chyle prepared by the digestive organs, poured into 

 the veins and mixed with the blood, is, thnjugh the me- 

 dium of the heart, sent into the lungs to be acted upon 

 by the air ; so the nutrimental juices of plants, taken up 

 from the earth, which has been called their stomach, are 

 carried by the sap- vessels into the leaves, for similar 

 purposes already mentioned. The improved sap, like 

 the vivid arterial blood, then proceeds to nourish and 

 invigorate the whole frame. I very much doubt, how- 

 ever, if those who suggested the above hypothesis^ 

 could have given so satisfactory an explanation of it. 



That the secretions of plants are wonderfully constant 

 appears from the operation of grafting. This consists 

 m uniting the branches of two or more separate trees, as 



