ORGANIZATION OF THE STEM. 51 



necessarily thickened and well charged with assimilated mat- 

 ters, and is interposed between the wood and bark where 

 growth is going on. " It is quite wrong/' says Dr. Gray, " to 

 suppose that there is any real interruption between the wood 

 and the bark at this or any other period of time, leaving a 

 space filled with extravasated sap. A series of delicate slices 

 will at any time show that the bark and wood are always 

 organically connected -with each other, by a very delicate tissue 

 of vitally active partly grown cells." The cambium thus 

 deposited between the wood and bark becomes organized into 

 cells, and forms a new addition of matter to each. Hence, the 

 forming stratum is termed the cambium layer, the inner por- 

 tion of which forms wood, and the outer, bark. It is when 

 this process of growth is most rapidly going on, in spring or 

 early summer, and the whole cambium layer is gorged with 

 the flow of sap, that the bark and wood are so easily separated. 

 But the separation is effected by the rending of a delicate new 

 tissue. 



At the end of the second year, the cambium layer of new 

 wood and bark hardens, the second annual layer or ring of 

 wood and bark is formed, and the bark and wood again adhere 

 firmly together. The new shoots are prepared for winter in 

 precisely the same manner as the first year's shoot was pre- 

 pared, and are elongated cones as was the first. 



In like manner will the plant continue to grow throughout 

 the third, fourth, and succeeding years, each annual growth 

 being only a repetition of the same phenomena. 



After a certain number of years the tree arrives at the full 

 perfection of its growth, the outer layers of bark now become 

 fissured and rent, and are exfoliated or thrown off from the 

 stem, and the alburnum or sapwood becomes changed into 



