i6o 



FERriLIZAriOiX OF THE OVUM 





D. Fertilization in Plants 



The investigation of fertilization in the plants has always lagged 

 somewhat behind that of the animals, and even at the present time 

 our knowledge of it is less complete, especially in regard to the 

 history of the centrosome and the archoplasmic structures. It is, 

 however, sufficient to show that the process is here essentially of 

 the same nature as in animals in so far as it involves a union of 



two germ-nuclei de- 

 rived from the two 

 respective sexes. 

 Many early observers 

 from the time of 

 Pringsheim ('55) on- 

 ward described a con- 

 jugation of cells in 

 the lower plants, but 

 the union of gcnii- 

 nnclci, as far as I can 

 lind, was first clearly 

 made out in the flow- 

 ering plants by Stras- 

 burger in 1877-8, and 

 carefully described 

 by him in 1884. 

 Schmitz observed a 

 union of the nuclei 

 of the conjugating cells of Spirogyra in 1879, and made similar obser- 

 vations on other algce in 1884. The same has been shown to be true 

 in MuscinccE. and Ptcridophytcs by Strasburger, Cambell, and others 

 (Fig. 79). 



Up to the present time, however, the only thorough investigation 

 of fertilization has been made in the case of the flowering plants, 

 and our knowledge of the process here is due in the first instance to 

 Strasburger ('84, '88) and Guignard (91), supplemented by the 

 work of Belajeff and Overton. The ovum or oosphere of the flower- 

 ing plant is a large, rounded cell containing a large nucleus and 

 numerous minute colourless plastids from which arise, by division, the 

 plastids of the embryo (chromatophorcs, amyloplasts). The ovum 

 lies in the " embryo-sac," which represents morphologically the female 

 prothallium or sexual generation of the Pteridophyte, and is itself 

 embedded in the ovule within the ovary. The male germ-cell is here 

 non-motile, and is represented by a "generative nucleus," with a 



Fig. 79. — Fertilization in Pibularia. [Cambell.] 

 A. B. Early stages in the formation of the spermatozoid. 

 B. The mature spermatozoid; the nucleus lies above in the 

 spiral turns; below is a cytoplasmic mass containing starch- 

 grains (cf. the spermatozoids of ferns and oi Marsilia, Fig. 53). 

 D. Archegonium during fertilization. In tlie centre the ovum 

 containing the apposed germ-nuclei (d", ?). 



