BOOK II. GENERAL CONSIDERATIONS. 239 



old bark and wood are separated in the spring by the exudation 

 from both of them of the glutinous, slimy substance called 

 cambium; wliich appears to be expressly intended, in the 

 first instance, to facilitate the descent of the subcortical fibres 

 of the growing buds ; and, in the second place, to assist in 

 generating the cellular tissue by which the horizontal dilata- 

 tion of the axis is caused, and which maintains a communica- 

 tion between the bark and the centre of the stem. These 

 lines of communication have, by the second year, become suf- 

 ficiently developed to be readily discovered, and are in fact 

 the medullary rays spoken of in the last book. It will be 

 remembered that there was a time when that which is now 

 bark constituted a homogeneous body with the pith ; and that 

 it was after the leaves began to come into action that the 

 separation which now exists between the bark and pith took 

 place. At the time when they were indissolubly united they 

 both consisted of cellular tissue, with a few spiral vessels upon 

 the line indicative of future separation. When a deposit of 

 wood was formed from above between them they were not 

 wholly divided the one from the other, but the deposit was 

 effected in such a way as to leave a commvmication by means 

 of cellular tissue between the bark and the pith ; and, as this 

 formation is at all times coaetaneous with that of the wood, 

 the communication so effected between the pith and bark is 

 quite as perfect at the end of the third year as it is at the 

 beginning of the first ; and so it will continue to be to the 

 end of the growth of the plant. The sap which has been 

 sucked into circulation by the unfolding leaves is exposed, as 

 in the previous year, to the effect of air and light ; is then re- 

 turned through the petiole to the stem, and sent downwards 

 through the bark, to be from it either conveyed to the root, 

 or distributed horizontally by the medullaiy rays to the centre 

 of the stem. At the end of the year the same phenomena 

 occur as took place the first season : wood is gradually de- 

 posited by slower degrees, whence the last portion is denser 

 than the first, and gives rise to the appearance called the 

 annual zones : the new shoot or shoots are prepared for winter, 

 and are again elongated cones, as vvas the first ; and this latter 

 has acquired an increase in diameter proportioned to the 



