223 
The Anatomy of Dioon edule, Lindl. 
of the plant turned a deep purple-red colour on exposure to the 
air. Previous to dissection the piece was boiled for some hours 
in order to soften the parenchymatous tissues of the cortex 
and medulla. Parts were preserved in spirit for the investigation 
of the minute anatomy; hand sections were cut and stained with 
iodine green and eosin. 
Anatomy. 
7 .—-Vegetative Axis. 
A transverse section of the stem shews a very large central 
medulla; this contains, as do all the other parenchymatous 
tissues of the plant, a great number of ramifying mucilage cells. 
Surrounding the medulla is the single vascular cylinder. This is 
a narrow zone of tissue, consisting of xylem, phloem and a middle 
layer of cambium ; the average thickness of the vascular layer is 
\ cm., i.e., about one-fifteenth of the radius of the stem. The vascular 
supply appears quite inadequate compared to the bulk of the stem 
(Pigs. 35 and 36. vas. cyl. I .); but the slow rate of growth, the small 
number of leaves functional at one time, and the xerophilous habit, 
necessitate a much smaller conducting channel than is usual in 
the majority of plants. The vascular system is a coarse irregular 
reticulum, pierced by medullary rays of breadth varying from one 
to about ten cells. 
From the protoxylems at the bases of the meshes of the 
network arise the leaf-traces (Fig. 35, l.t. I .); these pass out 
in the rays and take an oblique course in the vertical radia^ 
Fig. 35. Dissection of a hemi-cylindrical piece of the trunk ; vas. cyl. I., main 
axial cylinder; vas. cyl. II., inner vascular dome passing into the 
peduncular cylinder ; l.t. I., leaf-trace from the main axial cylinder; 
g.b., girdle-bundle; l.b., leaf-base; cor., cortex; med., medulla. 
