EXOGENOUS STRUCTURE. 121 



202. An herbaceous stem does not essentially differ from a 

 woody one of this age, except that the wood forms a less dense and 

 thinner zone ; and the whole perishes, at least down to the ground^ 

 at the close of the season. But a shrubby or arborescent stem 

 makes provision for an addition to its fabric the second year, 

 which may now be considered. 



203. Cambium-layer, The wedges which constitute the woody 

 layer usually increase in thickness throughout the season, by the 

 continued development of prosenchymatous cells on their outer 

 face, and the medullary rays extend equally by the multiplication 

 of parenchymatous cells : so that there is always a thin stratum of 

 delicate forming and growing cells interposed between the wood 

 and the bark. This is called the CAMBIUM-LAYER (Fig. 159, 

 160, C). It survives the winter in all exogenous stems capable of 

 more than one year's growth, remaining latent during the suspen- 

 sion of vegetation, and resuming its activity in the spring, to give 

 rise to 



204. The Second Tear's Growth in Diameter, In spring, when vege- 

 tation vigorously recommences, and the buds are developing the 

 onward growth of the season, a portion of the sap, charged with 

 mucilage (dextrine, protoplasm, &c.), is at the same time attracted 

 into the cambium-layer, as into every part where growth is going 

 on ; and the bark, before adherent, is now readily separable from 

 the wood. To this mucilaginous organizable matter the name of 

 Cambium was long ago applied, and hence the forming stratum is 

 termed the cambium-layer ; but the latter is only an incipient new 

 woody layer ; and the cambium is nothing more than ordinary 

 sap, well charged with dissolved assimilated matters, accumulated 

 at the part of the woody stem where further growth alone takes 

 place, and serving as the materials for such growth. It is quite 

 wrong to suppose that there is a real interruption 1 between the 

 wood and the bark at this, or any other period, leaving a space 

 filled with extravasated sap. A series of delicate slices will at any 

 time show that the bark and the wood are always organically con- 

 nected, by a very delicate tissue of vitally active, partly grown 

 cells, just in the state in which they multiply by division (26, 32). 

 It is when this process of growth is most rapidly going on, in 

 spring or early summer, and the whole cambium-layer is gorged 

 by the flow of sap, that the bark is so easily separable ; but the 

 separation is effected by the rending of a delicate new tissue. The 



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