86 BOTANICAL GAZETTE [AUGUST 
never seen a specimen of Dioon from which all of the armor had 
scaled off, except in case of injury. 
CorTEX.—The cortex of a large plant grows rapidly for a few 
years, and then during the long life of the plant grows very little. 
In a 6-meter specimen of Dioon spinulosum, at a distance of 15 cm. 
below the apex, the width of the zone of cortex, measured from the 
outer border of the phloem to the beginning of the leaf base region, 
was I-1.5 cm.; while the width of the cortex near the base of the 
stem, where the tissues were at least 100 years older, had increased 
only to 1.5 or 2 cm. 
_ Except in cases of injury, there are no meristematic regions in 
the cortex, Dioon being strictly monoxylic, and there is no growth 
by a phellogen at the periphery, the phellogen layers being confined 
to the leaf base region and not reaching the cortex itself. In forms 
which lose their armor through the vigorous activity of successive 
phellogens, the cortex itself is invaded, but in such cases the invad- 
ing phellogen adds as much or more than it cuts off, and the stem 
may even increase in diameter. 
_ The cortex is traversed by numerous leaf traces, sonie of them 
direct and others forming the characteristic girdle. There are also 
numerous mucilage canals and cavities, some of them following the 
course of the bundles, but most of them being independent. 
Crystals of calcium oxalate are numerous, and tannin cells are so 
abundant that a freshly cut stem changes color in a few minutes. 
OUNT OF XYLEM.—The cycad stem has always been described 
as having a large pith and cortex, with a small zone of wood between 
them. 
According to BRoNGNIART (1), a specimen of Zamia latifolia 
(doubtless an Encephalartos), 1.5 meters in height and 20.5 cm. 
in diameter, had a pith 7.5 cm. in diameter, surrounded by a vas- 
cular zone 6 mm. in width, the xylem and phloem being of about 
equal thickness; beyond the phloem was a narrow cortex about 
8 mm. in width, followed by a broad zone of leaf bases 5 cm. in 
width 
A few measurements which we have made recently are given 
below, all measurements being made at approximately the greatest 
diameter of the plant. 
