XVI. 



CIRCULATION OF THE SAP. 



TTTE do not yet know all about the circulation 

 if of the sap. The causes of its transfer are 

 less simple than was at one time supposed. For a 

 long time there was believed to be some analogy 

 between the movements of the sap in plants and 

 the circulation of the blood in the arteries and 

 veins of animals. This view was held by Erasmus 

 Darwin, who found in the stem of Tragopogon 

 scorzonera (black salsify,) two circles of vessels, 

 the inner of which he believed to serve for the 

 ascent of the sap, and to correspond to the arteries 

 of animals, and the outer to serve for its descent, 

 and to correspond to the veins. Such views have 

 not yet entirely disappeared. Thus Joseph W. 

 Talbot, of Massachusetts, says in the Transactions 

 of the Massachusetts Horticultural Society for 1879, 

 page 13: "It will not be controverted that the 

 crude sap ascends from the roots through the sap- 

 wood to the extremities, terminating in the buds, 

 and thence passes into the cambium layer. It will 

 be remembered that the cambium layer envelopes 

 every living part, and separates or lies between the 



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