PHENOMENA OF EEPRODUCTION. 



157 



be developed into the embryo, is at first filled with a transparent fluid, but soon 

 presents transverse septa at the upper contracted part which forms the sunpensor ; 

 then a longitudinal septum is formed in the swollen part, which answers to the free 

 end ; on which free end is afterwards developed either one lobe, or two opposite 

 lobes (cotyledons), and the opposite end becomes the caulicle. 



All physiologists concur in the above, but there are different opinions as to the 

 part the pollen plays. Schleiden contended that the tip of the pollen-tube forms 

 the embryo by forcing inwards the membrane of the embryonic 

 sac, folding it around itself, and occupying its cavity, where it 

 speedily develops into the embryo. Thus, according to Schleiden, 

 the ovule is merely a receptacle, destined to receive the embryo, 

 to protect and nourish it, the true reproductive organ residing 

 in the anther. But a closer examination instituted by the most 

 skilful anatomists of the French school has repeatedly disproved 

 the existence of the embryonic vesicle 

 before the arrival of the pollen-tube. Never- 

 theless, it is certain that the pollen 



ma- 



750. CEnotUera longiflora. 

 Anatropous OTuIe, cut Terti- 

 cally at the fcoment of ferti- 

 lization, to show the pollen- 

 tube, the end of which is in 

 contact with the embryonic 

 sac ; within, at the top of 

 this sac, are two vesicles, one 

 oC which will wither, and 

 the other form the embryo 

 (mag.). 



751. Santalima. 

 Placenta bearing three nuclei, 

 whence issue thi'ee embryonic 

 sacs, which receive three 

 pollen-tubes (mag.). 



752. Vertical section of fig. 751 

 (mag.), showing two of the 

 embryonic sacs within and 

 outside the nucleus. 



753. Santalum. 

 Portion of nut, cut Terfcically, 

 to show the embryonic sac 

 which has burst the nucleus at 

 the bottom, and has ascended 

 to the pollen-tube, the free end 

 of which itsheaths (mag.). 



terially assists in the formation of the embryo by means of its fovilla, which passes 

 by endosmose from the pollen-tube into the interior of the ovule. 



The fertilization of the ovule in SantalacecB presents a quite exceptional 

 phenomenon, which deserves to be mentioned (figs. 751,. 752, 753). The ovary is 

 unilocular, and the free central placenta bears several suspended ovules ; each is a 

 naked nucleus (without primine or secundine). At the period of fertilization, the 

 nucleus n bursts at the lower part, the embryonic sac se emerges by this opening 

 and ascends along the whole length of the outer surface of the nucleus, to meet 

 the pollen-tube tp a little below the top of the nucleus. The latter soon withers, 

 and the embryonic sac, which alone grows, forms the integument of the seed. 



