HICK AND CASH : AFFINITIES OF LEPIDODENDRON. 
325 
In the sporophyll is a single median vascular bundle, and one 
can generall}^ distinguish 'i.) the peculiarly developed basal portion, 
to which the sporangia are attached, and (ii.) the kniina, which is 
usually turned up at a sharp angle over the outer end of the sporan- 
gium. 
Heterospory is a common phen(nnena in the fruits of Lepido- 
dendron. Binney, Hooker, and others have described specimens in 
which both macrospores and microspores are present, and others will 
be found, both figured and described, in Williamson's Memoirs. In 
the Lepidostrohiiji dealt with by the last-named author in Part IIL, 
it is pointed out that the macrosporangia occupy the lower part of 
the fruit and the miscrosporangia the upper, as is the case with some 
living species of Selaginella, and other investigators have observed 
the same distribution. 
But although heterospory is common, homospory is not unknown, 
as some fruits have been found in which only microspores are present. 
Many of these however are b;it fragments, and hence it is possible 
that if the whole were at hand the other form of spore might be 
found. Moreover the possibility that some Lepidodendra were 
dioecious must not be overbooked, since the knowledge possessed does 
not enable us to maintain its untenability. Still, without minimising 
the importance of these considerations, it will be the safer course to 
regard some of the Lepidodendra as being homosporous. 
Stem of Lycopodium. 
In all species of the genus Lycopodium the stem has the same 
fundamental structure. 
The centre is occupied by an cLvik vascular cylinder, circular in 
cross section, formed by the fusion of several concentric or radial 
vascular bundles, and this is surrounded by a relatively thick cortejc. 
The xyleni portions of the cylinder consist of small groups of 
spiral vessels, between which are plates (jf scalariform traclieides. In 
fully developed stems" the whole vascular C3'linder is composed of 
xylem and phloem, so that there is no pith. 
In Lycopodium chainacyprissus the xylem is in the form of rib- 
bons which run more or less parallel to the dorsal surface of the stem, and 
the course of development of each is centripetal. In species with 
