88 



THE planter's guide. 



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 phjtological writers 

 we now have, is no ready granter of propositions ; yet 

 in his late work, while he holds the balance of decision 

 with an able and even hand between the conflicting theo- 

 ries, 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 pur- 

 pose nature has wisely provided such trees as are in open 

 exposures with a thick and coarse covering of outer bark, 

 which forms a defence 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 

 vegetation, as well as to continue it. Hence the wonderful 

 effects of shelter in close woods and plantations in encour- 

 aging growth. All' trees, during infancy, require a con- 

 siderable proportion of warmth to make them shoot freely, 

 as is proved by comparing the striking difference in their 

 progress at different degrees of elevation or exposure. 

 What is most remarkable 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. 



In adverting to heat as essential to vegetation, it is 

 particularly worthy of notice, as already observed, that 

 the epidermis and bark of trees drawn up by shelter are 



* Note II. 



