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74 ON THE SEXUAL OKGANS OF THE CYCADACE^.
ciilar layer is not generally completely exposed ; there still adheres an
excessively thin layer of cellular tissue which may belong (as has been
already stated) to the epithelial layer of the nucleus, or perhaps is
formed of the first cells of the endosperm.
It is generally known that the nucleus of Cycads, which is more or
less ovoid, is united for about two-thirds of its height with the coat,
but that it terminates above in a free summit, which is more or less
couical (Hooker calls it the cone in Weiwitschia). This free portion
is sometimes higher, sometimes shorter, but generally it is only slightly
projecting at first, and rises as the nucleus grows, so as to have its
summit sometimes immediately below the tubular exostome. Ordi-
narily conical in form, it is sometimes abbreviated, sometimes pro-
longed into a tube. When the endosperm increases in breadth, it is re-
duced to a lower level, but in the latter period of the formation of the
seed, the endosperm pushes it completely upwards, and in this case it
presents itself under an entirely ditferent form. Near its base the cone
is intimately united w^ith tiie coat, the internal surface of which, where
it is not united with the nucleus, is lined with a kind of epidermis,
which ultimately forms a brown and smooth layer. Externally the
cone is covered with a layer of dense cells (Schleiden's epithelium of
the nucleus, Grundzuge, ii. p. 349 ; Gotlsche's covering of the nuclear
protuberance, 1. c. p. 380). This becomes less distinct towards the
base, on the part of the nucleus which is adherent to the coat, but may
be recognised on the surface of the nucleus after boiling. Although
this layer, at the point where the nucleus becomes free, is intimately
united with the free internal surface of the coat, one cannot, especially
when taking the independent existence of the niicleus into considera-
tion, regard it as a continuation of the external epidermis ; «inoo-lliey«
j'^J< would thuo bo a structure >>i addit i on- of wh'ch I know no other ana-
•^ logue.*
At the summit of the cone this layer rises a little above the internal
r
' tissue, and surrounds it like a ring (Plate XCI. fig. 7/, and fig. 8 and 9).
The apex of the summit which is not covered by this epithelium, and
on which the pollen-grains fall to complete their development, and
* I must on this point differ advisedly from Oudemnns (Archives Neerl. ii.
p. 395)^ Tho point is mo ro cvifk -wt on oomii timi p tho ovviloo of Coniforfl' ; see
fop-elample, Schaclit, Flora, o. 13ot. Zeit. 1855, pi. ii. and the numerous figures
'whicli occur in the writings of Hofmcister.