HISTORY OF THE GROWTH OF A TREE. 



123 



increase of the whole of the cellular sys- 

 tem of the stem takes place, and each bud 

 sends down or^anisable matter within the 

 bark and above the wood of the <dioot from 

 which it sprang : thus forming on the one 

 hand a new layer of wood, and on the other 

 a fresh deposit of liber. 



In order to facilitate this last operation, 

 the old bark and wood are separated in the 

 spring by the exudation from both of them 

 of the glutinous slimy substance called 

 cambium ; w r hich appears to be expressly 

 intended, in the first instance, to facilitate 

 the development of the subcortical tubular 

 tissue ; and, in the second place, to assist 

 in generating the cellular tissue by which 

 the horizontal dilatation of the axis is 

 caused, and which maintains a communi- 

 cation between the bark and the centre of 

 the stem. This communication has, by the 

 second year, become sufficiently developed 

 to be readily discovered, and is effected by 

 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 be- 

 gan to come into action that the separation 

 which now exists between the bark and 

 pith took place. At the time when the lat- 

 ter were indissolubly united they both con- 

 sisted 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 the}- were 

 not wholly divided the one from the other, 

 but the deposit was effected in such a way 

 as to leave a communication by means of 

 cellular tissue between the bark and the 

 pith ; and, as this formation, or medullary 

 ray, is at all times coetaneous with that of 

 the wood, the communication so effected 

 between the pith and bark is quite as per- 

 fect at the end of any number of years as 

 it was at the beginning of the first; and so 

 it continues to the end of the growth of the 

 plant. 



The sap which is drawn from the earth 

 into circulation by the unfolding leaves is 

 exposed, as in the previous year, to the ef- 

 fect of air and light ; is then returned 

 through the petiole to the stem, and sent 

 downwards through the bark, to be from il 

 either conveyed to the root, or diatri! 



horizontally by the medullary rays to the 

 centre of the stem. 



At the end of the year the same pheno- 

 mena occur as took place the first season : 

 wood is gradually deposited by slower de- 

 grees, whence the last portion is denser 

 than the first, and gives rise to the appear- 

 ance called the annual zones: the new 

 shoot or shoots are prepared for winter, and 

 are again elongated cones, and the original 

 stem has acquired an increase in diameter 

 proportioned to the quantity of new shoots 

 which it produced, new shoots being to it 

 now, what young leaves were to it before. 



IV. The third year all that took place 

 the year before is repeated ; more roots 

 appear ; sap is again absorbed by the un- 

 folding leaves; and its loss is made good 

 by new fluids introduced by the roots and 

 transmitted through the alburnum or wood 

 of the year before ; new wood and liber are 

 formed from matter sent downwards by the 

 buds ; cambium is exuded ; the horizontal 

 development of cellular tissue is repeated, 

 but more extensively; wood towards the 

 end of the year is formed more slowly, and 

 has a more compact character ; and ano- 

 ther ring appears indicative of this years' 

 increase. 



In precisely the same manner as in the 

 second and third years of its existence will 

 the plant continue to vegetate, till the pe- 

 riod of its decay, each successive year 

 being a repetition of the phenomena of 

 that which preceded it. 



V. After a certain number of years the 

 tree arrives at the age of puberty ; the pe- 

 riod at which this occurs is very uncertain, 

 depending in some measure upon adventi- 

 tious circumstances, but more upon the 

 idiosyncrasy, or peculiar constitution, of the 

 individual. About the time when this al- 

 teration of habit is induced, by the influ- 

 ence of which the sap or blood of the plant 

 is to be partially diverted from its former 

 courses into channels in which its force is 

 to be applied to the production of new indi- 

 viduals rather than to the extension of it- 

 self; about this time it will be remarked 

 that certain of the young branches do not 

 lengthen, as had been heretofore the wont 

 of others, but assume a short stunted ap- 

 H arance, probably not growing two inches 

 in the lime which had been previously suf- 



