CIRCULATION OF THE SAP. 105 



sap-wood and the bark. In this layer all the buds 

 originate, and all growth is made. Nothing grows 

 outside of it except the bark. As the buds expand 

 and form shoots, they carry their envelope, the 

 cambium layer, with them. As in animal life 

 there is no connection between the arteries and 

 veins except at the extremities, so the ascending 

 sap has no connection with the elaborated sap in 

 the cambium except at the extremities in the buds. 

 The leaves, which are the receptacles of the crude 

 sap, throw off superfluous matter, and in their 

 wonderful laboratory supply carbon and other 

 elements from the air, preparatory to its entrance 

 into the cambium layer, whence it passes down for 

 the building up and nourishing the whole tree." 



In " The Gardener's and Farmer's Reason Why," 

 a book published in London, in 1860, the author, 

 in answer to the question, What are the functions 

 of the bark ? says : 



" It serves to protect the young wood from 

 injury, and to act as a filter through which the 

 descending juices of a plant may pass horizontally 

 into the stem or downward to the root." 



The idea that the sap passes in a definite cur- 

 rent upward in the wood and downward between 

 the wood and bark after being " elaborated" in the 

 leaves is the popular belief at the present time. 

 The familiar example of a tree continuing to live 



