960 APPENDIX. BOOK HI. 



ovulum in Avicennia, 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 Thesium ebracteatum by Warming 

 (Ann. Sci. Nat., s6r. 6, t. V, 1878). One of the hypodermal cells enlarges and becomes 

 the archesporial cell : this is divided transversely into two, the lower becoming the em- 

 bryo-sac, the upper dividing transversely into two superposed tapetal cells. These tapetal 

 cells are peculiar 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 Orchid^es, Ann. d. Sci. 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, i88o. 



Page 629. On abnormal fibro-vascular bundles in Monocotyledons, see Kny, Ueb. 

 einige Abweichungen im Bau des Leitbiindels 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 III. 



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 Zellhaute, 1882), 

 Strasburger dissents entirely from Nageli'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 Nageli 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. 



Nageli'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 aff'ected by tension, strain, etc. It was from these facts that he inferred 



