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 phloém 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 may be regarded 
as homologous with the vessels of the wood, and, like 
those, are constituted of series of segments. Hach 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) 
