316 FERTILIZATION. 



that on reaching the embryo-sac it indents the latter, pushing it 

 forwards so as to reverse a portion on itself, in which cavity the 

 apex of the pollen-tube swells into an oval or globular form, and its 

 contents are transformed into new cells, which, as they grow and 

 multiply, shape themselves into the embryo. Or, according to other 

 observations, it is maintained that the apex of the pollen-tube pierces 

 the embryo-sac and developes into the embryo in its interior, in 

 the manner last stated. It is now unnecessary to adduce the de- 

 tails of the researches, or the theoretical considerations, by which 

 this hypothesis was supported. For, besides the researches of Mir- 

 bel, in 1839, the investigations made, between the year 1846 and 

 the present time, by Amici, Mohl, K. Miiller, linger (who had 

 maintained the hypothesis in question), Eloffmeister, Henfrey, and 

 Tulasne, have completely overthrown the foundations on which it 

 rested ; by proving, 1st. That the embryonal vesicle, from which 

 the embryo is developed, exists in the embryo-sac, in some cases 

 at least, before the pollen-tube has reached the ovule ; so that it 

 cannot owe its origin to the pollen-tube, directly or indirectly, and 

 still less can it be a prolongation of it. 2d. That end of the pol- 

 len-tube is, for the most part', at least, applied to the exterior of the 

 embryo-sac at a point distinguishably, and often considerably, dis- 

 tant from that where the embryo is developed within.* 



577. The general results which all these recent investigations 

 conspire to establish are these: The pollen-tube entering the 

 orifice of the ovule, penetrates the tissue of the nucleus until it 

 reaches the summit of the embryo-sac. Sometimes its extremity 

 slightly indents it ; often it glides downwards along the surface of 

 the sac for a little distance ; in either case it barely adheres to the 



* The latest memoir on this subject, that of Tulasne (in Ann. Sci. Nat for 

 July and August, 1849), is remarkable not only for its thoroughness and its 

 admirable illustrations, but because the author here points out and corrects 

 the error into which he had formerly fallen, which led him to conclude that 

 the end of the pollen-tube actually penetrates the embryo-sac, and gives rise 

 to the embryonal vesicle. Hoffmeister asserts (as rendered by Henfrey), 

 that although the pollen-tube generally rests upon the outside of the embryo- 

 sac, yet in a very few isolated cases it perforates it; but "even when the 

 pollen-tube thus penetrates into the interior of the embryo-sac, its end remains 

 perfectly closed, and the membrane of the germinal vesicle quite uninjured : 

 in no case can a direct passage of the contents of one into the other take place. 

 The impregnation is the result solely of an endosmotic exchange of the fluid 

 contents." Henfrey, Bot. Gazette, I. c. 



