DISPLACEMENTS DURING GROWTH. 



445 



another. Now if the medullary rays were exact orlhogonal trajectories of the annual 

 rings, as is the case where the growth of the wood is regular, they must cut the cir- 

 cumference of the disc of wood at the points r r. But all the medullary rays, on the 

 contrary, are curved towards the point N, and therefore away from S; or, in other 

 words, they are driven over towards the line of strongest growth, which reaches 

 from the pith to N. This scheme is intended to illustrate no theory, but only the 

 fact, which is very often to be observed on transverse sections of wood, that the 

 medullary rays are driven towards the side of strongest growth of the wood, and 

 therefore sacrifice their rectangular intersection with the pcriclines — i.e. the annual 

 rings. Fig. 282 represents a particularly clear example in the transverse section of an 

 excentrically grown stem of the Lime. It is easily noticed that the medullary rays s t 

 are driven from the right as well as from the left towards a, the line of strongest growth. 



It is not to be forgotten, however, that we are here concerned with a transverse 

 section of wood, the growth and cell-divisions of which take place exclusively in an 

 outer narrow zone of the periphery, viz. in the cambium, whereas in similar cases in 

 growing-points the entire mass of embryonic tissue with its anticlines and periclines is 

 growing. 



We have hitherto regarded the cell-network as a superficies only, and the 

 anticlines and periclines as simple lines. If we give a certain thickness to these flat 

 structures, nothing is essentially altered thereby ; what are mere areolae in the above 

 figures then behave like the stones of a mosaic, and are actual cells. The matter is 

 quite otherwise, however, when we come to look upon the figures hitherto considered 

 as longitudinal or transverse sections of ellipsoidal or spherical bodies, or when the 

 organ possesses any such form as that of a lens, or a compressed ellipsoid, &c. 



