a 
— 
SECONDARY THICKENING 61 
thickening differs usually quite materially from the pri- 
mary xylem. It contains much more fibrous tissue, is 
more compact, and forms a true wood. The phloem 
also is interspersed with more bundles of bast, and may 
by its formation soon crush out of recognizable shape the 
primary phloem. In addition, the tissues forming the 
primary medullary rays become active. The layer of 
parenchyma cells that connects the edge of the cambium 
of one bundle with that of the next bundle becomes 
itself converted into cambium by the accumulation of 
large amounts of cytoplasm in the cells, and the formation 
of periclinal walls. Part of this interfascicular cambium 
thus formed gives rise only to cortical and medullary 
parenchyma, but at intervals new bundles arise by the 
formation of xylem and phloem, respectively, on the 
inner and outer faces of the cambium layer. Thus, sec- 
ondary bundles are formed, which divide the medullary 
rays longitudinally, and as the bundles become more and 
more numerous, these primary rays may 
eventually be reduced to thin plates of paren- 
chyma, only one or two cells thick, and per- 
haps only a few cells wide (measured in ver- 
tical direction), but still extending from the 
pith to the cortex. Additional (‘‘second- 
ary”’) medullary rays are formed within the _ Fre. 29 — 
bundles when certain cambium cells cease growth ofsvas. 
to form xylem elements and from that time ° aera 
forward produce parenchyma cells. These secondary 
medullary rays usually arise at varying distances from 
the center, a certain number of new ones being laid down 
each year. 
92. Where the growth is continuous and equal, the 
wood is usually of fine grain and uniform. Most woody 
plants of the temperate zones, however, and of those 
