PART II. 



CHAPTER Vr. 



PROTOPLASM AND ITS RELATIONS TO ITS SURROUNDINGS. 



536. Upon the framework which imparts strength to the 

 plant the active, living cells are distributed. In old ligneous 

 dicotyledonous plants the living parts are relatively so super- 

 ficial that the}- have been said to form a mere film of living 

 tissue held in place b^- a dead skeleton.' 



537. The living csUs are those which contain protoplasm. 

 Each of these cells has definite relations to the neighboring cells, 

 most of which relations have been presented in Part I. But each 

 of these cells has also definite relations to the external world, 

 ■which it is the province of Ph^-siolog^- to investigate. Such an 

 investigation naturally begins with a consideration of the char- 

 acter of protoplasm. 



^ " The living parts of a tree or shrub, of the exogenous kind, are obviously 

 only these : 1st, The summit of the stem and branches, with the buds which 

 continue them upwards, and annually de-velop the foliage. 2d, The fiesh 

 roots and rootlets annually developed at the opposite extremity. 3d, The 

 newest strata of wood and bark, and especially the interposed cambium-layer, 

 which, annually renewed, maintain a living communication between the root- 

 lets on the one hand and the buds and foliage on the other, however distant 

 they at length may be. These are all that are concerned in the life and growth 

 of the tree ; and these are annually renewed. . . . The plant is a composite 

 being, or communitj', lasting, in the case of a tree, through an indefinite and 

 often immense number of generations. These are successively produced, en- 

 joy a term of existence, and perish in their turn. Life passes onward con- 

 tinually from the older to the newer parts, and death follows, with equal step, 

 at a narrow interval. No portion of the tree is now living that was alive a 

 few years ago ; the leaves die annually and are cast off, while the intemodes or 

 joints of the stem that bore them, as to their wood at hast, buried deep in the 

 trunk under the wood of succeeding generations, are converted into lifeless 

 heart-wood, or perchance decayed, and the bark that belonged to them is 

 thrown off from the surface. It is the aggregate, the blended mass alone, that 

 long survives" (Gray's Structural Botany, pp. 83, 84.)- ^ 



