APPENDIX. BOOK III. 
Ovulum in A'vicennia, Trans. Linn. Soc. XX, 1846). In the first place Treub points out 
that the ovule is not naked, as Griffith states, but that it has an integument developed from 
the dermatogen in the same way as that described for Theshim ebracteatum by Warming 
(Ann. Sei. Nat., ser. 6, t. V, 1878). One of the hypodermal cells enlarges and becomes 
the archesporial cell : this is divided transversely into tvi^o, the lovi'er becoming the em- 
bryo-sac, the upper dividing transversely into two superposed tapetal cells. These tapetal 
cells are pecuhar in that they persist for a considerable time, whereas in most plants they 
are absorbed before fertilisation. The embryo-sac enlarges, pushes aside the tapetal cells, 
and absorbs the epidermis at the micropylar end of the ovule. After fertilisation the 
embryo is soon to be seen surrounded by endosperm cells, and at one side of this group of 
cells there is a large cell, termed by Treub the ' cotyloid ' cell, which elongates towards 
the apex of the embryo-sac. The endosperm now grows and projects through the micro- 
pyle, until finally it is quite external to the ovule. It still encloses the embryo, but as the 
embryo grows it ruptures the endosperm and the cotyledons project, the radicular end 
remaining inserted in the endosperm. During this time the cotyloid cell has enlarged, 
branching posteriorly in the ovule and penetrating anteriorly into the placenta. It acts as 
an absorptive organ, taking up nutritive substances from the ovule and the placenta, and 
transmitting them to the endosperm and thus also to the embryo. The radicular end of 
the embryo is peculiar in that it is destitute of a root-cap ; but previously to the dehiscence 
of the fruit adventitious roots, generally four in number, provided with root-caps, spring 
from it close to its attachment to the suspensor. 
Page 603. On the flowers of Orchids, see Gerard, Sur I'Homologie et le Diagramme 
des Orchidees, Ann. d. Sei. Nat., s^r. 6, t. VII. 
Page 611. On the symmetry of the flower, see also Eichler, Ueb. einige zygo- 
morphe Bliithen, in Sitzber. d. Ges. naturf. Freunde, Berlin, 1880. 
Page 629. On abnormal fibro-vascular bundles in Monocotyledons, see Kny, Ueb. 
einige Abweichungen im Bau des Leitbündels der Monokotyledonen, Sitz. d. Bot. Ver. d. 
Prov. Brandenburg, 1881. 
Page 650, lines 5 and 9 from bottom ; for ' secondary cortex ' read ' secondary phloem.' 
Page 653. On abnormal modes of thickening of the stem in Dicotyledons, see 
Appendix, p. 950. 
BOOK m. 
Page 663. The Condition of Aggregation of organised structures. In his work on 
the structure and growth of the cell-wall (Ueb. Bau und Wachsthum der Zellhäute, 1882), 
Strasburger dissents entirely from Nägeli's theory of the structure of organised bodies 
which is given in the text. A short account of his views may be given here. 
Strasburger comes to the conclusion that the forces which hold together the solid 
particles of organised bodies are of a chemical, as opposed to a physical, nature. The 
chemical molecules are not grouped together into micellae by cohesion, and the micellae 
are not connected into organised substance by attraction, as Nägeli would have it, but the 
molecules are linked together by chemical affinity, probably by means of multivalent atoms, 
into networks. Further, the water present is retained, according to Strasburger, in the 
intermolecular meshes by capillarity. The phenomenon of ' swelling-up ' is therefore one 
of intermolecular capillarity, and depends upon the mobility of the molecules about their 
position of equilibrium. 
Nägeli's micellar theory received considerable support from his observations upon 
the appearances presented by organised bodies (starch-grains, cell-walls) when examined in 
polarised light. He found that they were doubly refractive, and further that their double 
refraction was not affected by tension, strain, etc. It was from these facts that he inferred 
