290 BOTANICAL GAZETTE 3 [OCTOBER 
strand. It is to be noted that the megasporangial tracheids 
described by TrREUB and BENSON in Casuarina, Castanea, Corylus, 
etc., are equally without any relation of continuity with the bundle 
system of the ovule. In general, in the angiosperms the opening 
of the cavities of the anther takes place by means of a mechanical 
device, which is not of epidermal origin, and consequently cannot 
be regarded as an annulus. The structure in question has clearly 
originated in the deeper tissues of the sporangial wall, and is 
in all probability derived from the fibrovascular system, as are 
similar devices in Ginkgo and the Abietineae. In the highest seed 
plants all relation between the opening mechanisms of the anther 
and the fibrovascular structures have apparently long disappeared. 
It is obvious from the discussion of the pertusing devices of 
sporangia in the preceding pages that there are two main types 
of opening mechanisms; namely, the annulus, which is clearly of 
epidermal origin (in the Cycadales actually containing stomata), 
and, secondly, the fiber layer, occurring in a more or less complete 
condition from the Ginkgoales upward. This seems equally clearly 
to have been derived from the fibrovascular structures. Those 
sporangia which are opened by the instrumentality of an annulus 
may be appropriately designated ectokinetic, while those which owe 
their dehiscence to the stresses originated by an internal mechanical 
system, derived from the fibrovascular tissues, may with equal 
fitness be termed endokinetic: If these distinctions be well founded, 
they obviously supply us with a valuable additional criterion for 
the course of evolution in the higher plants. The application of - 
these criteria makes it apparently clear that the angiosperms can- 
not have been derived from the Cycadophyta, a phylogeny which, 
moreover, meets many other anatomical difficulties and cannot 
even be easily reconciled with a reasonable interpretation of the 
external features of the reproductive parts of the bennettitalean 
Cycadales and the ranalian angiosperms. 
Summary 
1. In Ginkgo the opening mechanism is clearly an adjunct of 
the transfusion tissue of the fibrovascular system and is directly 
continuous with this. 
