MEDULLARY RAYS. 223 
The elementary organs of the wood,’ as the vascular portion 
of stems may be briefly termed, are as follows: the vessels and 
tracheids, or tubes conveying water, the wood-parenchyma cells, 
which serve for conducting the carbo-hydrates, and the libriform 
cells, or the specifically mechanical elements of the wood. Be- 
sides these, the wood is traversed in a radial direction by the 
medullary rays. 
The first three forms of cells have already been considered. 
The libriform cells* (wood-cells, wood-fibres) are the bast- 
cells of the wood (Figs. 137, 139), and therefore, strictly con- 
sidered, belong to the mechanical system (see the latter). They 
are of service, however, especially in the transitional forms, to the 
other elements of the wood, occasionally also for conducting and 
storing nutritive material, and are therefore sometimes provided 
with contents. They are prosenchymatous, thick-walled, 
provided with cleft-like, oblique pores, and are never as long. 
as the true bast-cells. 
The medullary rays (parenchyma rays) are aggregates of cells 
consisting of one or several rows, always built up of cells which 
are very much extended radially (Figs. 78, 137, 144, 149), and 
which pass from the medulla through the cambium and the 
sieve portion, often penetrating deeply into the bark (Figs. 100 
r, 118 m, 149, 137 r, 180, 181).* 
When the interior of stems is filled with fundamental tissue, 
the so-called medulla, it is thus, by means of these radiating 
lines of cells, placed in connection with the tissue of the outer 
bark, which is located on the periphery of the fibro-vascular 
bundles. These rows of cells are therefore very appropriately 
called medullary rays. Their development in a vertical direc- 
tion is governed by the number of rows of cells placed over each 
i i tilized by 
'The anatomy of the wood has been diagnostically u 
Nérdlinger (* Anatomische Merkmale deutscher Wald- und Garten- 
holzarten,” Stuttgart, 1881). : 
? Liber, bast, forma, form. Compare Sanio, “ Vergleichende Unter- 
suchungen tiber die Elementarorgane des Holzkérpers,” Bot. Zeit., 1863, 
101. 
® Compare also Berg’s “ Atlas,” xxxvi. to xl., Figs. 86-94 r. 
‘ 
