66 GRAFTING. 



found to contain a greater proportion of mucilage at 

 the end of autumn 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 animals, and occasioning, through their 

 means, the dispersion of the seeds ; and the perfume 

 of flowers may attract insects, and .so promote the fer- 

 tilization 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, through 

 the medium 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 pro- 

 ceeds to nourish and invigorate the whole frame. I 

 very much doubt, however, if those who suggested the 

 above hypothesis, could have given so satisfactory an 

 explanation of it. 



That the secretions of plants are wonderfully con- 

 stant, appears from the operation of grafting. This 



