112 THE OAK. 



ing mass of wood drives it outwards, on the other. The 

 other differences chiefly concern the individual 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 cor- 

 tex. 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 ele- 

 ments of the secondary phloem are as follows : 



(1) Bast parenchyma (Fig. 17, b p) is developed, like 

 the wood parenchyma, from cambium cells which un- 

 dergo a few transverse divisions and then pass over as 

 longitudinal groups of cells, which retain their living 

 contents, etc. From these longitudinal groups, accom- 

 panying the sieve-tubes as parallel series, they are called 

 companion cells (cambiform cells). 



(2) Sieve-tubes (Fig. 18, bp], which may be 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 sieve) 



