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70 ON THE SEXUAL OKGANS OF THE CYCADACE.E.
edges. The same structure may be recognized in the biovular carpo-
phylls of other Cycadacere, and we see that in them the flattened form
gives rise to tetragonal forms in consequence of the opposing mutual
pressures.
6. Anatomically the ovule resembles a thickened leaf-segment, in
wliich tlie tissues are arranged round a centre instead of being drawn
out in a plane.* I pointed out this homology in 1842 (Monog. p. 13),
and Heinzel (Diss, de Macrozamia) has taken a similar view.
The external layers of the carpophyll are composed of parenchyma,
becoming more merenchymatous in the interior. At the same time
elongated cells with thickened walls frequently appear in this region.
The same arrangement of the tissues occurs in the coat of the ovule in
every Cycad which I have examined, ^'he two layers, as I have else-
where shown, ^ and as is now generally admitted, § form morphologi-
cally only a single coat. TiiC external coat, which is filled with juices
later on, is green when young, but frequently coloured red when ma-
ture. The internal layer represents the more prosenehyraatous part of
the carpophyll. It soon becomes woody, the points where afterwards
what are called the sutures occur, becoming so last. The two layers
are reduced to their least thickness in the tubular exostome at the
summit ; || both play a more or less important part in the formation of
* M. Casimir de Candolle in a recent paper on the theory of the leaf
(Archives des Sciences, May, 1868, translated in 'Student,' Aug. 1868), considers
leaves as branches with tlie side turned towards the axis undeveloped.
The ovules of Cycads may be looked upon as reversions to a more complete
structure of particular portions of tlie carpophyll.—W. T. D.
t As in otlier plants, it then anatomically diverges a little from the leaf (see
above, and Kraus in Pringsheim's Jahrb. t. iv).
I " Structura inlegumenii peculiaris est, et ab ilia ovulorum, qualia hucus-
que novimus, aliquomodo diversa. Inde ab initio oU'ert :
" 1. Stratum exlernum carnosum, cellulis parencliyniatici.s rcgularibus con-
fiatum, eadem epidermide ac carpophyllum vestitum . . . . , apex hujus strati
tubulosus.
"2. Stratum secundum, ligneum vel osseo-lignenm, cellulis parenchymaticis
et clongatis compositum, materia deposita inde a prima origine lignescentibus."
I quote this passage because an entirely different opinion has been recently
attributed to me (C. A. J. A. Oudemans in Vers, en Meded. der Koninkl.
Akad. vol. ii. p. 255, et Arch. Neerl. vol. ii. p. 395). The fact that the two
layers are entirely Mended and are developed simultaneously is suiBcient to
show the absurdity of regarding them as two distinct coats.
§ See Eiclder in Martiu.=