250 



REPRODUCTION, VEGETABLE (VEGETABLE OVUM). 



teen cells, of which eight are superficial, and 

 the other eight enclosed as a central spheri- 



Fig. 193. 



Later stage. Division of parent-cell of embryo by a 

 vertical septum. 



The vesicles contained in the upper tubular por- 

 tion of the germ-cell have now arranged them- 

 selves so as to form a filamentous prolongation 

 to the embryo, about 200 diam. 



cal mass. By the frequent repetition of the 

 same process it increases in size, still retaining 



Fig 19-k 



Isolated slxteen-celled embryo of the same, with its 

 filamentous prolongation, about 150 diam, 



its globular form, until it is transformed into 

 an embryo, the direction of growth of the 

 axis of which is downwards. 



107, Orchis Morio. In the Orchideae the 

 structure of the ovule is remarkably simple. 

 The following description of the mode of origin 

 and early development of the embryo, in 

 Orchis Morio, all the stages of which we have 

 ourselves followed, is taken from Mr. Hen- 

 frcy's paper on Vegetable Reproduction, in the 



Annals of Natural History : " The ovule 

 springs from the placental surface as a single 

 projecting cell, which, by subdivision, soon 

 becomes a cellular papilla (the nucleus), com- 

 posed of a central cell (the embryo-sac), sur- 

 rounded by a simple cellular layer. The two 

 coats gradually grow up over this, and by the 

 greater elongation of one side the ovule 

 becomes anatropous. The nucleus mean- 

 while loses its cellular coat, apparently by 

 absorption, and appears as a large oval sac 

 enclosed in the coats, consisting in fact merely 



195. 



Early condition of ovule of Orchis mascula. 



The embryo-sac is exposed in consequence of the 

 absorption of the cells which previously sur- 

 rounded it, 180 diam. 



of an embryo sac. In the apex of this, 

 about the epoch when the pollen falls upon 

 the stigma, three cellules (embryonal vesicles), 

 make their appearance at the upper end of 



Fig. 196. 



Isolated embryo-sac of the same immediately before 

 impregnation, containing three embryonal vesicles, 

 180 diam. 



the embryo sac, formed apparently by free 

 cell-formation around a globule of protoplasm. 

 The pollen masses on the stigma send down 

 pollen tubes, which traverse the conducting 

 tissue of the style, and make their way to the 

 placentas, where they enter, ordinarily, singly 

 (sometimes more than one) into the micro- 

 pyle canals of the ovules, and come in contact 

 with the outside of the apex of the embryo 

 sac, immediately above where the embryonal 

 vesicles lie ; but the pollen tube does not pene- 

 trate the embryo sac. Soon after the pollen 

 tube has reached the embryo sac, one (very 

 rarely two) of the embryonal vesicles begins to 

 swell, becomes divided by a cross septum into 

 two cells, and while the upper one grows out 

 in a filamentous form through the micropyle, 

 by a continued process of cell-division, the 

 lower cell enlarges, and divides repeatedly so as 

 to form a cellular globule the embryo, which 

 in this plant does not go on to produce a co- 

 tyledon and radicle, as in most other cases. 

 The filamentous prolongation, the use of 



