50 THE INNER ORGANIZATION OF TREES. 



take part in its vital operations, and therefore still remain- 

 ing a necessary part of it, as a living physiological whole. 



It is also evident that the different species of cells which 

 united together form the tissues of trees, like their leaves 

 and other organs, are individuals having distinct physio- 

 logical functions to perform. These plant-cells differ from 

 the phytons or leaves only in the greater simplicity of their 

 organization ; both are governed by the same general laws, 

 having a life peculiar to themselves, and their own period 

 of arriving at a state of maturity and decay. There is also, 

 evidently a similar division of organic labor amongst the 

 cells, as amongst the leaves ; each cell contributes its part 

 to the building up of the tree, and the separate and com- 

 bined labor of the whole of them is necessary in order to 

 effect those various transmutations and changes of the raw 

 nutritive material, or sap, into its final products. We know 

 that some of the cells, such as the fibre and duct-cells, are 

 principally employed in carrying the raw material from one 

 part of the cell community to the other, and that these 

 changes are wrought in the parenchyma cells, not of the 

 leaves alone, but in parenchyma everywhere ; for the paren- 

 chyma cells of the root and pith are frequently as rich in 

 starch as those of the bark or leaves. But we are at 

 present profoundly ignorant not only of the nature of these 

 transmutations, but also of the order in which they take 

 place : for that the entire series are governed by fixed laws 

 of sequence is plainly indicated by the whole of the vital 

 phenomena of the plant. 



We see it put forth a regular series of organs, which 

 follow each other in a determinate order of time, and not 

 only contribute to the nourishment of each other, but yield 

 up life in succession in effecting a progressive metamor- 

 phosis of the constituents of the sap, and the advance of the 

 plant towards the perfection of its structure. And first the 

 seed-covers are ruptured and the nursing-leaves atrophied 

 in the development of the first series of phytons ; these latter 

 die in developing the stem and the buds on its outside. These 

 buds consist of a series of covering leaves, which shelter the 



