30 Mitchell.—Contributions towards a Knowledge of the 
of the sporangium, which, although small and insignificant here, corresponds 
essentially to the elaborate boat-shaped structure in the megasporangium. 
On drying, the sporangium opens at the top with a sudden jerk, 
the flaps separating quickly from each other. Each layer of thickened cells 
acts independently (strongly reminding one of the fern annulus), small spore 
masses being shot off by the snapping of different parts of the sporangium 
wall. The majority of the spores are, however, still unshed when the flaps 
have reached their maximum divergence, half lying poised on each flap. 
This condition is maintained for a time, the approximation of the small 
basal walls again causing the shedding of the spores, which are in this case, 
however, only just thrown clear of the sporangium wall, to a distance 
of i to i J cm. The microsporangia resume their normal shape with great 
rapidity on placing them in water, but I have not been able to induce them 
to perform vigorous movements on drying a second time. 
The thickenings in the microsporangium are of a far more general 
character than those in the megasporangium, and it is easily seen how the 
specialized structure of the megasporangium may have been evolved from 
the simpler type of microsporangium by localization of the thickening 
in the basal region to form a powerful ‘ slinging ’ mechanism, and the 
development of the accessory ‘ hinge * tissue and crack regions to render 
more easy and effective the ejection of the spores. 
The Vascidar System. 
The gross anatomy of the strobilus can best be seen on dissection, 
after boiling the cones in dilute caustic potash, on which it becomes evident 
that the vascular system here is essentially simpler than it is in the 
vegetative axes. All the species which I have examined (save .S'. Lyallii , 
which will be referred to later) have a single vascular cord, with, typically, 
two marginal protoxylems (although S. Bakeriana has four, and S'. spmosa 
eight), from which are given off at regular intervals the spirally arranged 
leaf-traces. The stele is suspended in a definite lacuna, which broadens out 
considerably at the leaf base, ending abruptly just underneath the spo¬ 
rangium. It is traversed by the usual trabeculae, with the characteristic 
endodermal thickening peculiar to each species. The central core of xylem 
consists usually of well-thickened scalariform tracheides, but there may be 
a more or less well-marked procambial area extending from the tip down¬ 
wards. The protoxylem elements are fairly large spiral and delicate 
annular tracheides, as are also those of the leaf-traces, which are always 
simple (save in the case of S'. Lyallii , where occasional splitting takes place 
into two bundles), slender at first, but broadening out in the lamina of the 
leaf, and finally ending abruptly in a tuft of tracheides. 
Frequently, as in 5 . Wildenowii , S'. helvetica , &c., the leaf-trace shows 
a slight enlargement almost immediately on leaving the central strand. 
