130 Worsdell . — The Structure and Origin of the Cycadaceae . 
the resemblance in habit to a Fern ceases, unless it be in the Fern-like 
foliage of the genera Stangeria and Bowenia. The male and female 
reproductive organs are borne on cones, each such cone being developed as 
a direct continuation of the main axis ; the growth in length of the plant 
is subsequently continued by means of a lateral vegetative shoot im- 
mediately below the cone, this latter being pushed to one side. The main 
mode of branching of the plant is thus sympodial. But monopodial 
branching also occurs where, as is quite frequently the case, short, lateral 
shoots are developed at intervals on the main axis. The plants are dioecious. 
In the case of the female plant of Cycas there is no cone developed, the 
seed-bearing sporophylls arising in groups at intervals directly on the main 
axis, this latter persisting in growth and developing time after time foliage- 
leaves above the zone of sporophylls. This is probably a primitive 
ancestral condition, the cone-formation representing a phylogenetically 
younger character. 
In all Cycads whorls of foliage- and scale-leaves are formed alternately 
on the vegetative stem. 
It would appear that the tap-root of the plant early dies away, its 
place being taken by adventitious roots which arise from the stem-tissues. 
This also is a Fern-like character. 
All Cycads are xerophytes, i.e. are adapted to withstand the con- 
ditions of drought and intense solar heat and light under which they grow ; 
hence the armour-plate of thick leaf-bases on the stem, and the rigidity of 
structure and thick cuticularization of the leaves. 
Let us turn now to the anatomical structure of the vegetative stem. 
The tissues are extremely parenchymatous and contain an abundance of 
starch. Mucilage-canals traverse both the wide pith and the cortex, 
passing also through the medullary rays of the vascular cylinder. They 
are continuous into the leaf, but not, as a rule, into the root. Periderm is 
present in the leaf-bases and outer cortex. 
Of the nine genera of Cycads, four, viz. Cycas, Encephalartos , Macro - . 
zamia , and Bowenia , possess more than one vascular ring. In Cycas there 
may be as many as a dozen rings arranged concentrically one within the 
other. In any case, only the innermost of these rings arises in the first 
place as primary tissue with secondary cambial formations subsequently 
increasing its radial thickness. All the other vascular rings are entirely 
of secondary origin, being formed by the agency of a cambium. The re- 
maining 1 genera, viz. Stangeria , Zamia , Ceratozamia , and Dioon possess 
but a single ring in their stems. Both the xylem and phloem of each are 
extremely parenchymatous, being largely composed of very numerous 
medullary rays. This vascular tissue belongs to the collateral type of 
structure. The leaf-trace bundles, as they enter the stem to the number 
1 But Microcycas has never yet been examined. 
