PLANT TISSUES 139 
pits and medullary-rays make up the wood of Conifers and most 
of the other orders of Gymnosperms. 
The functions of tracheids are longitudinal conduction of 
crude sap and support to the part containing them. 
MEDULLARY Rays 
These are bands of parenchyma cells which extend radially 
from the cortex to the pith (primary 
medullary rays) or from a part of the . 
xylem to a part of the phloem (sec- 
ondary medullary rays or “‘vascular 
rays”). Secondary medullary rays © 
occur only as a result of the activity 
of the cambium during secondary 
growth of the plant axis. The xylem 
portion of a medullary ray is termed 
a xylem ray or wood ray while the 
phloem portion is termed a phloem ray. oy ove a 
In _ tangential-longitudinal section 6 © Molle@ 
medullary rays usually appear 
: : a Fic. 77.—Portion of radial- 
- : e in 
epacie shapes (Fig aP icy longitudinal section through 
radial-longitudinal sections they are cq of White Pine stem. {, 
seen crossing the other elements. tracheid; », bordered pore of 
Their primary function is to supply ee medullary —_ ray. 
the cambium and wood with elabo- ~*8"™"" 
rated sap formed in the leaves and conveyed away by the sieve 
tubes and phloem parenchyma and to supply the cambium 
and phloem with crude sap which passes up mainly through the 
tracheze and tracheids from the absorptive regions of the roots. 
They distribute food materials radially. They furthermore 
with cells longer and more pointed and walls becoming thickened and pitted; 4, 
complete wood fibers with walls more thickened than in the previous stage and 
lignified, as shown by the stippling. The protoplasts in this last stage have disap- 
peared and the fibers are dead. D, steps in the formation of wood parenchyma 
from cambial or procambial cells. 1, Group of cambial or plerome cells; 2, the 
same enlarged in all dimensions; 3, the same with walls thickened and pitted; 4 and 
5 show the same stages as 2 and 3, but here the cells have enlarged radially or 
tangentially more than they have vertically. The walls of these cells are apt to 
become lignified, but the cells are longer lived than the wood fibers. (From 
Stevens.) 
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