PREFACE. 



slipping and grafting in tliose trees that admit of 

 being propagated in that way. The other waSj re- 

 flection on the entire absence in all vegetables of 

 anything corresponding to the renewal of substaiice^ 

 which is constantly going on in the living tissues of 

 animals^ and by which alone their continued vitality 

 is maintained. Once in possession of these two 

 keys^ the whole theory^ as here unfolded^ gradnally, 

 and without any sensible effort, took shape in his 

 mind. The difficulty referred to vanished^ and the 

 lawSj imagined to be exceptional, stood out in their 

 character of universality. 



Trees being in themselves objects of universal 

 interest — of interest even to the young, the author, 

 while he has made it his first aim to treat his subject 

 in a scientific spirit, has yet striven also to present it 

 in a form, and in language adapted to the general and 

 the juvenile reader. And, holding to the young per- 

 sons to whom these Letters are more immediately 

 addressed, the relation therein indicated, and writing 

 under the promptings of that relation, he has not 

 hesitated; as occasion offered, to introduce such ob- 



