125 



known ; in Lathraa squamaria it was observed in the form of 

 shapeless excavations. However, according to my observa- 

 tions, the sac is not constantly situated in the interior of the 

 nucleus, and in fact M. Schleiden's description of the struc- 

 ture of the ovule previous to impregnation cannot be consi- 

 dered as a general rule, by the conception of which every case 

 of deviation would appear very easily explainable. In nume- 

 rous cases the sac of the embryo is only formed subsequent to 

 impregnation ; and in others, where the nucleus occurs long be- 

 fore in the form of a hollow sac consisting of a simple cellular 

 layer, it then occupies itself the place of the embryonal sac. 



M. Schleiden lastly arrives at the actual act of impregna- 

 tion, the plastic processes of which have hitherto been de- 

 scribed in the writings of Amici, Brongniart, Rob. Brown, and 

 Corda. On the 31st of July 1833, Robert Brown made known 

 that he had followed the pollen tubes in Asclepias phytolac- 

 coides to the apex of the ovule, and therefore to the summit of 

 the nucleus, since in Asclepias this is perfectly naked, which 

 Mr. Bro\vn did not then venture to advance : " to this point/* 

 it is stated in that short paper, ee the tube adheres so firmly that 

 I am inclined to think it actually penetrates, to some depth 

 at least, into the substance of the ovulum," &c. But on Mr. 

 Brown's visit to Berlin, in the first days of September 1833, 

 he had already advanced somewhat further in this inquiry, for 

 he showed me with his accustomed kindness the entrance of 

 the tubes into the summit of the nucleus, which was after- 

 wards seen in Breslau also, although Corda subsequently took 

 no notice of it. In a memoir which M. Corda presented to 

 the Kais. Leopold. Akademie* in September 1834, he gave 

 his observations on the act of impregnation in the Conifera ; 

 he saw the pollen tubes ascend through the cavity of the se- 

 cundine to the opening of the nucula, enter this, become 

 thinner in its cavity, and discharge their contents in the form 

 of a turbid liquid mass. This mass M. Corda observed soon 

 to fashion itself, forming the embryo-sac, but the connexion 

 of the pollen tubes with the embryo-sac continues for some 

 time after impregnation. 



I have prefixed these historical statements in order to render 



See the Jahresbericht for 1835. Wiegmann's Archiv, 2 tem Jahrg. 2 ter 

 Band. 



