55 



can we suppose any one to imagine, that 

 there is a irue circulation of the sap of 

 plants, like that of the blood of animals. 

 By the circulation of the sap is meant 

 merely its ascent through the wood into 

 the leaves and buds, thence into the green 

 outer pith of the bark, on which the leaves 

 and buds are situated, and its descent to 

 the roots, through the rings of pith of the 

 bark. How the descent dies off and stops, 

 it is difficult to imagine ; but it is still 

 more difficult to suppose that any part of 

 the sap should re-^ascend. The whole 

 affair, however, is a matter of the merest 

 conjecture. And the fate of the physio- 

 logist is to build theories whose very data 

 are doubt and difficulty. 



The proper juices " of plants are found 

 in this green " herbaceous envelope for 

 example, resin in the fir ; and the woods 

 of different trees do not differ more in 

 their proper constituents than the barks 

 of different trees ; and, possibly^ as the first 



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