68 FLOWERING PLANTS 
Fig. 37 is a diagram showing the tissues in a young root, as 
already described. In this the bast alternates with the wood, 
and there is no cambium. ) 
Fig. 38 represents, also diagrammatically, a section through 
that part of the root which has begun to turn brown and 
37 38 39 

Figs. 37-39. DIAGRAMS ILLUSTRATING FORMATION OF SECONDARY 
Woop AND Bast In DIcoTYLEDONOUS Roots. 
p.b, primary bast ; p.w, primary wood ; pi. r, primary medullary rays ; 
s.m.7, secondary medullary rays ; sec. w, secondary wood ; see.b, secondary 
bast ; cort, cortex ; p./, piliferous layer ; c, cambium. 
bears many lateral roots. In this the four groups of primary 
wood (p.w) still alternate with the groups of primary 
bast (p.b), as before. Between the bast and the wood cells 
undergoing division are seen ; this is the cambial layer, which 
18 giving rise to secondary bast (see.b) and secondary wood 
(sec.w). This layer (c) arises in the pericycle opposite the 
wood, then spreads round outside the wood and inside the 
bast, so that first of all it forms a ring of wavy outline. It 
divides exactly as the cambium of the stem does, giving rise 
to secondary bast towards the outside, secondary wood towards 
the inside, and medullary rays (sm.r) along certain radii. 
The cambium opposite the original wood groups gives rise 
entirely to medullary tissue, so that large broad medullary 
rays extend from the groups of primary wood to the pericycle. 
Fig. 39 represents a transverse section of a root taken still 



