CAUSE OF BLEEDING. 51 



of the root, the bleeding, as a natural consequence, was greatest 

 at that part. He also remarked that bubbles of air were 

 frequently formed on the cut surface, evidently showing that 

 some kind of gas was present, either in the sap or in the 

 cells'. The discharge was perfectly visible to the naked 

 eye, and in bright weather the microscope enabled him to see 

 distinctly the downward passage of the sap, through all the 

 root cells. 



Such cases are doubtless . much more common than is 

 supposed. The cause of the phenomena was in part demon- 

 strated more than a century since by Hales. In discussing 

 the question of the circulation or non-circulation of the 

 sap, this great experimentalist uses the following words ; — 

 "We see in many of the foregoing experiments, what 

 quantities of moisture trees do daily imbibe and perspire. 

 Now the celerity of the sap must be very great, if 

 that quantity of moisture must, most of it, ascend to the 

 top of the tree, then descend, and again ascend, before it is 

 carried off by perspiration. The defect of a circulation in 

 vegetables seems in some measure to be supplied by the much 

 greater quantity of liquor which the vegetable takes in, than 

 the animal, whereby its motion is accelerated ; for by Experi- 

 ment 1st, we find the Sunflower, bulk for bulk, imbibes and 

 perspires seventeen times more fresh liquor than a man every 

 twenty-four hours. Besides, Nature's great aim in vegetables 

 being only that the vegetable life be carried on and maintained, 

 there was no occasion to give its sap the rapid motion which 

 was necessary for the blood of animals. In animals, it is the 

 heart which sets the blood in motion, and makes it continually 

 circulate ; but in vegetables, we can discover no other cause of 

 the sap's motion but the strong attraction of the capUlary sap 

 vesselsj assisted by the brisk undulations and vibrations caused 

 by the sun's warmth, whereby the sap is carried up to the top of 

 the tallest trees, and is there perspired off through the leaves : 

 but when the surface of the tree is greatly diminished by the 

 loss of its leaves, then also the perspiration and motion of the 

 sap is proportionably diminished, as is plain from many of the 

 foregoing experiments ; so that the ascending velocity of the 



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