DEVELOPMENT OF VEGETABLES. 75 



of the bark, which had been first formed, and which yield 

 to this pressure to a certain extent; but, becoming tlicm- 

 selves consolidated by the effects of the same pressure, they 

 acquire increasing rigidity; and, the same cause continuing 

 to operate, they at length give way, in various places, form- 

 ing those deep cracks, which are observable in the bark 

 of old trees, and which give so rugged an appearance to 

 their surface. The cuticle has, long before this, peeled off, 

 and has been succeeded by the consolidated layers of corti- 

 cal envelope which form the epidermis. But the epider- 

 mis, which is continually splitting by the expansion of the 

 parts it encloses, itself soon decays, and is constantly suc- 

 ceeded by fresh layers, produced by the same process of 

 consolidation in the subjacent cortical substance. 



During the third and each succeeding year, the same pro- 

 cess 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 ver- 

 tically, while they are capable of elongation, and can be sup- 

 plied with nourishment. 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 farther dilatation 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 inhalntants of the forest; till, arriving at 

 maturity, its majestic form towers above all the junior or 

 less vigorous trees. ^' 



The development of each branch takes place in the same 

 manner, and by the same kind of process, as that of the trunk. 

 The buds from which they originate, spring from .the angle 



* It is contended by Dr. Uarwin and other writers on vegetable physiology'' 



that each annual shoot should be rcg-arded as a collection of individual buds, 



each bud being a distinct individual plant, and the whole tree an aggregation 



V of such individuals. I shall have occasion to revert to this question when I 



come to consider the subject of vegetable nutrition. 



