THEORIES OF EMBRYOLOGY. 



245 



Wydler, Tulasne, Gelesnow, and Wilson. Schleiden, who has made 

 a very elaborate series of observations on the embryo, is in favour of 

 this view. According to him, the 

 extremity of the pollen-tube does not 

 enter the embryo-sac, but continues 

 on the outside of it, pressing in the 

 sac before it, and thus becoming sur- 

 rounded by a double layer of it. The 

 end of the pollen-tube (fig. 436 e) 

 forms the germinal vesicle, in the 

 interior of which nuclei and cells are 

 produced, which ultimately give origin 

 to the embryo. All the portion of 

 the pollen-tube within the embryo-sac 

 may be developed as the embryo, or 

 a portion may remain in the form of 

 a suspensor, or suspensory filament, 

 attached to the upper part of the sac. 



509. Wydler, Gelesnow, and Tulasne, maintain that there is no 

 indentation of the sac, but that the tube enters it directly. Schleiden 

 thinks that this may be true in certain cases where the embryo-sac 

 becomes elongated upwards, and then the membrane is absorbed so as 

 to allow the pollen-tube to penetrate into the interior. Schleiden thus 

 looks upon the embryo as a foreign body entering from without, and 

 supports his views by cases of polyembryony in Coniferse, Cycadaceae, 

 Misletoe, Onion, &c., where the plurality of embryos, according to him, 

 depends on more than one pollen-tube having entered the foramen of 

 the ovule. Wilson has adopted Schleiden's views, from observations 

 made on Campanula rotundifolia. Tulasne, from examining the 

 embryogeny of Veronica hederifolia, triphyllos, and prsecox, concurs 

 in Wydler's views. Endlicher supports similar views with Schleiden, 

 considering the stigma, however, as an organ, the peculiar secretion of 

 which acts on the pollen-grain so as to render it capable of penetrating 

 to the ovule, and developing an embryo. Unger's opinion nearly cor- 

 responds with this. Griffith, from his researches on Viscum, Santalum, 

 Osyris, and Loranthus, concludes, that the pollen-tube penetrates the 

 embryo-sac or cavity, and passes through it longitudinally ; and he 

 seems to think that, in some cases, the embryo proceeds from cells 

 developed from the end of the tube. Klotszch states that pollen-tubes 

 may be seen entering the embryo-sac in Lavatera tremestris, Tobacco, 

 and some Orchidaceag, after the pollen has lain from twenty-four to 

 thirty-six hours on the stigma. Hofmeister admits that the pollen- 



Fig. 436. Section of ovule to illustrate Schleiden's view of fertilization, r, Raphe. c *, 

 Chalaza. p, Primine. s, Secundine. e x, Exostome. n, Endostome. n, Nucleus, e *, Em- 

 bryo-sac, p t. Pollen-tube, e, The embryo formed by the extremity of the pollen-tube 



