FUNCTION OF NUTRITION. 227 



CHAP. IV. 



FUNCTION OF NUTRITION CONTINUED Period 7- 



ASSIMILATION (223.). PRUNING (225.). GRAFTING (227.). 



DEVELOPMENT (230.). NUTRITION OF CRYPTOGAMIC 



PLANTS (233.). PARASITIC PLANTS (234.). DURATION OF 



LIFE (235.). VEGETABLE INDIVIDUALS (236.). LONGE- 

 VITY OF TREES (239.). 



SEVENTH PERIOD OF NUTRITION. 



(223.) Assimilation. THE chief end and object of 

 the various processes which we have been describing, is 

 the manufacture of the materials which are ultimately 

 to be assimilated into the vegetable structure, and by 

 which it is to be nourished and developed in all its 

 parts. Of the precise manner in which the assimilation 

 of this nutriment takes place we know nothing, and 

 the first steps towards the formation and development 

 of any organised being are entirely concealed from us. 

 We may indeed observe when a gradual organisation of 

 matter is taking place; but there is no stage in the 

 process from whence we may not refer back to some 

 previous state, out of which it appears to have emerged 

 imperceptibly and inexplicably ; and it is utterly im- 

 possible to note with any degree of accuracy, either the 

 precise manner or exact time when the first traces of 

 any new condition of organisation commenced. In other 

 words, as soon as we can distinguish an organ it already 

 exists in a developed form, however faintly its subor- 

 dinate parts may be indicated. 



(224.) Growth of the Tissues In dicotyledonous 

 trees, as we have observed (art. 34. 2.), the new tissue 

 makes its appearance between the old wood and old 



