OVULES. 



283 



great advantage of this view is that it serves to homologize the 

 fructification of Flowering Plants with that of the higher Flower- 

 less Plants, or the Ferns, the sporangia or analogues of the 

 ovule being outgrowths of the leaf. 1 



532. Origination of the Embryo. The whole process of fer- 

 tilization and the resulting produc- 

 tion of the embryo, also the history 

 of the subject, belongs to the suc- 

 ceeding volume, involving as they 

 do questions of minute anatomy and 

 of physiolog}'. But a general idea 

 may here be given of the way in 

 which the embiyo originates. The 

 tube which a grain of pollen sends 

 forth into the stigma (574, 575) 

 penetrates the style through loose 

 conducting tissue charged with 

 nourishing liquid, reaches the cavity 

 of the ovary, enters the orifice of an 

 ovule to reach the apex of the nu- 

 cleus, although the latter sometimes 

 projects to meet the pollen-tube. 

 Meanwhile a cavity (the embryo- 

 sac, which is formed by the great 

 enlargement of a single cell of the 

 tissue, or of two or more cells the 

 product of a mother cell) forms in 

 the nucleus, the upper part of it 

 commonly reaching nearly or quite 

 to the apex of the nucleus, which 

 the pollen-tube impinges on or 

 sometimes penetrates. A particular portion of the protoplasm 

 contained in the embiyo-sac forms a globule, and this at the time 



1 The advocates of this view naturally maintain that ovules and placentae 

 always belong to leaves, and never truly to a cauline axis ; that in the pro- 

 central placentation of Primulaceae, the actual ovuliferous surface is an out- 

 growth of the bases of the carpellary leaves coalescent with each other and 

 adnate to a prolongation of the torus ; also that in those Gymnosperms which 

 have no earpophyll, such as Yew, the whole nascent carpellary leaf, or rather 

 the papilla which would otherwise develop as such, is directly developed into 

 ovule. This, being solitary and the last production of the axis, necessarily 

 appears to terminate it. (500, 501, notes.) 



FIG. 598. Diagram representing a magnified pistil of Buckwheat, with longitudinal 

 section through the axis of the ovary and orthotropous ovule ; some pollen on the stigmns. 

 one grain distinctly showing its tube, which has penetrated the style, reappeared in the 

 cavity of the ovary, entered the mouth of the solitary ovule (o), and reached the 

 embryo-sac (s) near the embryonal vesicle (v). 



