122 



siderable ingenuity ;* and both Du Hamel and Hales, while 

 they generally admitted that the sap both ascends and de- 

 scends, denied the existence of a circulation. Within the 

 last five-and-twenty years, however, the theory has been re- 

 vived with great lustre, and seems now to be the popular one 

 of the day. In fact, from the arguments as well as names, 

 by which it has been supported, it bids fair to stand its ground 

 in future. Mr. Keith, one of the clearest and best phytolo- 

 gical writers we now have, is no ready granter of proposi- 

 tions ; yet, in his late work, while he holds the balance of 

 decision with an able and even hand, between the conflicting 

 theories, he very nearly admits the existence of the doctrine.! 



From this cursory account it is apparent of what vast 

 importance it is to the planter to maintain the sap, and still 

 more the proper vessels, in the due exercise of their functions, 

 and to protect them from external injury, of which cold may 

 be considered as the greatest. For this purpose, nature has 

 wisely provided such trees, as are in open exposures, with a 

 thick and coarse covering of outer bark, which forms a de- 

 fence from the elements to the inner bark, in which the 

 descending or proper vessels are situated. 



Further : We know that heat is necessary to cause vege- 

 tation, as well as to continue it. Hence the wonderful effects 

 of shelter, in close woods and plantations, in encouraging 

 growth. All trees, during infancy, require a considerable 

 proportion of warmth, to make them shoot freely, as is proved 

 by comparing the striking difference in their progress, at dif- 

 ferent degrees of elevation or exposure. What is most re- 

 markable in sheltered trees is, that several of the kinds, most 

 delicate and tender while young, for example the oak, are 

 found, when matured in a kindly temperature, to be the best 

 adapted to resist the elements, and set their greatest fury at 

 defiance. 



* Phys. des Arbres, L. V. ch. 2. f iNotf 11. 



