200 Dr. H. Karsten on the Sexual Life of had good reason for supposing, with Ehrenberg, that the car-bonate of lime of which they are composed was derived from decayed Foraminifera ; but at the same time a strict proof would have been wanting, and we might have adopted the opinion ex-pressed by Hai dinger in his paper on the Metamorphism of Rocks *, and concluded that, though, according to Ehrenberg, chalk does contain very many organic bodies, it does itself con-sist of rounded forms, which are a chemical deposit from water containing soluble salts of lime. Now, however, that their real origin appears to be established, it is no longer requisite to assume the existence of any unknown crystalloidal force differing from simple crystallization ; and we can clearly perceive that, though presenting characteristic differences, chalk is in every respect analogous to what we should have, if the mud now being formed at great depths in the Atlantic, by the accumula-tion of various minute organic bodies, were to be subsequently more or less altered by molecular changes or chemical actions of a well-known character. There is, however, one striking differ-ence ; for the Atlantic mud contains many Diatoraacese, spicula of Sponges, and other silicious organic bodies, which are very rare in, or absent from, the chalk : it contains, however, sili-cious concretions ; and this contrast in the state and aggrega-tion of the silicious matter in the two otherwise analogous depo-sits makes me very much inclined to conclude, with Ehrenberg f, that the silex of the flints was derived from disseminated silicious organic bodies, which has collected round various centres of segregational attraction, — though there are some difficulties to remove before that opinion can be finally adopted. XX. — On the Sexual Life of Plants, and Parthenogenesis. By Dr. H. Karsten, Lecturer on Botany at the University of Berlin. [Concluded from page 99.] Embryogeny. The elongated pollen-cells on the stigma of Ccelebogyne ilici-folia exhibit no peculiarity in the onward course they pursue to the nucleus of the ovule. The amylum and the vesicles with nitrogenous contents (mucus-vesicles) become dissolved as the pollen grows ; and when the pollen-tube has reached the large embryo-sac, it is seen to be filled with fluid, which also in all probability contains a number of vesicles, freely swimming about in it, some with and others without nuclei. * Haidinger's Wiener Mittheilungen, 1848, iv. 103; Neues Jahrbuch iiir Mineralogie, 1849, 213. t Abliandlungen d. k. Ak. d. Wiss. zu Berlin, 1838, 82.
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