64 THE CONICAL GROWTH OF TREES. 



the last three years, are the most developed ; to these is therefore 

 to be attributed the three broad wood-rings. 



The above investigations would seem to lead irresistibly to 

 the conclusion, that the breadth of the wood-rings is determined 

 not only by the activity of the leaves of the terminal shoot of 

 the axis, but that the leaves of the side axes, or of the whole 

 system of axes, co-operate, and therefore that the leafage of 

 each season forms a common source, whence is derived not only 

 the nutriment forming the new layer or covering of each indi- 

 vidual branch or system of axes, but of the main axis or support 

 of the whole of them the stem. For not only each leaf, but 

 each twig, branchlet, and branch contributes its part, during the 

 season of vegetative activity, to the formation of this new coni- 

 cal layer or woody mantle, with which the tree is annually 

 re-clothed from the tops of its branches to the extremities of its 

 roots, a new garment of unity, so to speak. 



A clear conception of the entire process of this annual reno- 

 vation, can only be obtained by giving due prominence to the 

 fact, that the growth and life of the tree after the first year is 

 entirely superficial, and totally confined to the forming stratum 

 of new bark and wood. The bark and wood cells constituting 

 the growth of each year, die when their vital activity ceases in 

 the Fall. There is no renewal of their life on the return of 

 Spring. For, as we have already stated on page 44, " So soon 

 as a cell ceases to form new cells, or to develope and carry nour- 

 ishing matter, so soon as its fluid contents disappear and it 

 becomes filled with air, it may be considered as dead." Now, 

 this is precisely the condition of the duct cells, and to a certain 

 extent of the fibre cells, at the end of the first year. They are 

 fully formed the first year, and when the life of the tree is 

 reawakened in the Spring of the next year, they no longer 

 grow, or assimilate formative material. They are dead, rigid, 

 unyielding. For the sap is drawn away from these old and full- 

 grown tissues, by the young and newly -forming stratum of bark 

 and wood, and although its flow through the finer capillary ves- 

 sels of the fibre cells may be again renewed, this flow arises 

 from causes purely mechanical, there is no assimilation of 

 formative material. In fact, the only change that takes place 



