ORIGIN OF THE EMBRYO. 305 



proved in all points by a series of elaborate investigations made hy 

 Mirbel, Amici, Giraud, Mold, Hofmeister, Unger, Tulasne, Henfreyj 

 and Radlkofer. So that — passing by the whole history of this long 

 discussion, and merely appending some references to the more im- 

 portant publications upon the subject* — we need only state here, 

 in the most general terms, the principal facts which are now held 

 to be established, viz. : — 



579. The pollen-tube terminates on the outer surface of the 

 embryo-sac, or sometimes, perhaps, forces its way into it. Ordina- 

 rily its extremity becomes firmly adherent to the surface of the 

 embryo-sac, and it appears to remain closed. Henfrey, indeed, is 

 led to suppose that the membrane of the pollen-tube and that of the 

 embryo-sac are absorbed at the point of contact, and that the foi-mer 

 thus discharges its contents into the cavity of the latter ; but this is 

 merely an unproved inference, suggested by the analogy of what is 

 now known of the process of fecundation in Cryptogamous plants. 

 At present it appears most probable that the contents of the pollen- 

 tube are drawn into the embryo-sac by endosmosis. However this 

 may be, shortly after reaching the embryo-sac the pollen-tube be- 

 comes empty, and decays or withers away. Meanwhile the body 

 which by its development is to give rise to the embryo appears in 

 the embryo-sac independent of the pollen-tube. According to most 

 investigators it generally appears before the pollen-tube has entered 

 the ovule. (The high authority of Tulasne, however, is thus far 



* Schleiden first published his famous theory in "Wiegmann's ArcTiiv, 1837, 

 and in Acta Nova Acad. Nat. Cur., Vol. 19. It was extended and defended in 

 his systematic works, — and especially by Schacht in Trans. Netherlands Insti- 

 tute, 1850, in Bot. Zeitung, 1855 (transl. in Ann. Sci. Nat. of tliat year), in his 

 Beitriige Anat. ^ Pht/s., in his worlc on the microscope, of which an English 

 translation by Dr. Currey was published in 1855, and in the Eegensberg 

 Flora, 1855 (Ann. Sci. Nat. 1855). See also Deecke in Bot. Zeitung, 1855 

 {A)in. Sci. Nat., 1. c.). On the other side of the question the most important of 

 the recent publications, since the appearance of Mohl's Principles of the Anatomy 

 and Physiology of the Vegetable Cell, in the English translation (1852), and the 

 article Ovule in the Micrographic Dictionary by Henfrey, are : Hofmeister, in 

 Flora, May, 1855, and Mohl, in Bot. Zeitung, June, 1855 (both reproduced in 

 Ann. Sci. Nat., ser. 4, Vol. 3, 1855) ; Tulasne, in Ann. Sci. Nat., ser. 4, Vol. 4, 

 1855, being the complement of his great memoir published in the same journal 

 (ser. 3, Vol. 12, 1849) ; Radlkofer, Die Befruchtung der Phanerogamien, Leipsic, 

 1856 ; Henfrey, Development of the Ovule of Sanlalum album, &c., in Trans. Linn. 

 Soc, Vol. 22, part 1, 1856. 



26* 



