112 THE OAK 



thickening mass of wood drives it outwards, on the 

 other. The other differences chiefly concern the in- 

 dividual elements now to be described. 



All that was said of the medullary rays in the wood 

 applies also to those in the bast ; the cambium in keep- 

 ing open or originating new medullary rays does so on 

 both sides, and therefore the medullary rays are to be 

 traced radially through the cambium from wood to 

 cortex. The rays in the bast are termed ' bast rays ; ' 

 the broader ones contain isolated groups of sclerotic 

 cells and cells containing crystals. 



The changes which the radial rows of cells on the 

 exterior of the cambium zone undergo to form the 

 elements of the secondary phloem are as follows : 



(1) Bast parenchyma (fig. 17, ~bp) is developed, like 

 the wood parenchyma, from cambium cells which undergo 

 a few transverse divisions and then pass over as longitu- 

 dinal groups of cells, which retain their living contents, 

 &c. From these longitudinal groups, accompanying the 

 sieve-tubes as parallel series, they are called companion 

 cells (cambiform cells). 



(2) Sieve-tubes (fig. 18, bp), which maybe regarded 

 as homologous with the vessels of the wood, and, like 

 those, are constituted of series of segments. Each seg- 

 ment corresponds to a cambium cell, and is obliquely 

 tapering at the end where it fits on to another segment. 

 These dividing septa are not completely broken through, 

 as in the case of the wood-vessels, however, but are 

 pierced by a grating-like series of holes (the sievej 



