DEVELOPEMENT OF VEGETABLES. 89 



same process of consolidation in the subjacent 

 cortical substance. 



During the third and each succeeding year, 

 the same process is repeated ; new layers of 

 cellular texture and of woody fibres are deposited 

 around those of the preceding year's growth, and 

 a new internal coating is given to the liber of the 

 bark. The compressing power continues to be 

 exerted on the internal layers of wood, directing 

 their growth vertically, while they are capable 

 of elongation, and can be supplied with nourish- 

 ment. In time, however, by continued pressure, 

 and accumulating depositions of solid matter, 

 the vessels and the cells become less and less 

 pervious to fluids ; till at length all further dila- 

 tation is prevented. But the tree still continues 

 to enlarge its trunk by the annual accessions of 

 vigorous and expansible alburnum, and to take 

 its station among its kindred inhabitants of the 

 forest; till, arriving at maturity, its majestic 

 form towers above all the junior or less vigo- 

 rous trees.* 



The developement of each branch takes place 

 in the same manner, and by the same kind of 



* It is contended by Dr. Darwin and other writers on vege- 

 table physiology that each annual shoot should be regarded as 

 a collection of individual buds, each bud being a distinct indivi- 

 dual plant, and the whole tree an aggregation of such individuals. 

 I shall have occasion to revert to this question when I come to 

 consider the subject of vegetable nutrition. 



