DEVELOPEMENT OF VEGETABLES. 77 



pressure, they acquire increasing rigidity ; and, the 

 same cause continuing to operate, they at length 

 give way in various places, forming 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 consoli- 

 dated layers of cortical envelope which form the 

 epidermis. But the epidermis, which is continually 

 splitting by the expansion of the parts it encloses, 

 itself soon decays, and is constantly succeeded by 

 fresh layers, produced by the same process of con- 

 solidation 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 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 further dilatation is prevented. But the 

 tree still continues to enlarge its trunk by the 

 annual accessions of vigorous and expansible albur- 

 num, and to take its station among its kindred in- 

 habitants of the forest ; till, arriving at maturity, 

 its majestic form towers above all the junior or less 

 vigorous trees.* 



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

 pliysiology that each annual shoot should be regarded as a collec- 



