Comparative Anatomy of the Casuarineae . 241 
with the process which takes place in the fall of the leaf in 
autumn. 
In a transverse section of a still older woody stem the 
phloem at the periphery is seen to be composed of elements 
somewhat irregularly arranged, consisting of sieve-tubes with 
a rather narrow cavity, companion-cells (Fig. 11), and phloem- 
parenchyma. Some of the parenchyma-cells contain crystals 
of calcium oxalate. 
The cells of the pith are generally large, often pitted and 
lignified, thus probably assisting in the conduction of water. 
Many of the cells may contain starch. 
A considerable portion of the xylem is taken up by the 
medullary rays. These are of two kinds : there are rays 
consisting of larger cells and from two to several layers in 
tangential thickness, and other, more inconspicuous rays, con- 
sisting usually of only one layer of cells. Goeppert 1 was the 
first, to describe these rays, which have been sufficiently well 
studied by later authors. Both kinds of rays are very 
numerous, and their cells contain starch and crystals of cal- 
cium oxalate, &c. 
Lying between the rays, in regular radial rows, are the 
fibrous tracheides, composing the main portion of the wood. 
They usually have very thick walls ; but this character varies 
in different parts of the stem and in different species. As 
usually seen, they possess a wall about equal in thickness 
to the width of the cavity, and with one or two bordered 
pits connecting them with surrounding tracheal elements. 
In some species, zones of fibrous tracheides occur in which 
the walls have acquired such a thickness as entirely or almost 
entirely to obliterate the cell-cavity, while no pits of any kind 
are to be seen in the wall ; these elements have thus the 
general appearance of fibres, such as are typical for the 
higher Dicotyledons. In the outer portion of the wood of 
C. torulosa , Ait., there were zones of elements more irregularly 
placed with regard to one another which had far more the 
1 Loc. cit., p. 749. 
