THE VEGETABLE CELL. 129 



' nation resembling that we observe in the spore. Bat the pollen-grain 

 does not seem to be capable of a farther deyelopraent, under fayourable 

 external cireumstances^ into a plant like the parent, yet Reisseck and 

 Karsten observed that under certam circamstance&, e. ^., when pollen-grams 

 were enclosed in hollow stems like that of the Dahlia, their inner coat was 

 ca]}able of an abnormal development, and of conversion into lower forms 

 of Fungi. 



** The OduU. 



The Ovule {ovulwm, Elchen), — of late years called by the ad- 

 herents of Schleiden's theory of impregaation the seed-bud (samen- 

 knospe) or gemraule, consists essentially of a parenchymatous 

 papilliform growth from the ovary, of the so- 

 called nucleus (ei-kerne, nucleus ovuli fig. 52, Fig. 52. 

 a), the tercine of Mirbel, in which towards the 

 epoch of impregnation one cell becomes more 

 enlarged, displacing a greater or smaller portion 

 of the parenchyma of the nucleus and forming 

 the emhryo-sac (the quintine of Mirbel). 



In far tlie greater number of cases the ovule 

 does not stand still at this first stage, in wMcli 

 it consists merely of a naked nucleus, but un- 

 dergoes, before impregnation, a more or less 

 extensive series of changes, which relate partly Transverse section of 

 to the formation of enveloping, membranes, en- Imb^o^^'^rm^rioat 

 closinot the nucleus, and partly to alterations of of the ovuie 'iprmnm); 



f> V n. ^ ^ 1 jfii I'jy* ."» Outer Coat (,v<?m«- 



form dependent upon curvatures 01 the dinerent dine) ; e, Mwropyie ; /, 

 parts of the ovule. ^ ^ '''^"'^" ' ^; ^^"^^""^"«- 



The coats of the ovule originate in this way : at a variable dis- 

 tance from the summit of the nucleus, an annular collar of cells 

 makes its appearance, growing into a thicker or thinner coat which 

 gradually rises up round the nucleus and contracts over its apex 

 leaving only a little orifice, the micropyle (ei-miuncU, fig. 52, e)> 

 In the majority of ovules, a second coat (fig. 52, d) is formed in the 

 same way, lower down than the first (fig. 52, e) which it encloses. 

 That part of the ovule where the simple or double coat is connect- 

 ed with the base of the nucleus (fig. 52, /), is named the chalaza, 

 and when beneath this there exists a cylindrical portion, it is 

 called the funiculus (nahelstrang, fig. 52, g), 



Ohserv, Since the changes of form, which the ovules of most plants un- 

 dergo in the course of their development, exercise no influence upon their 

 impregnation, 1 shall be content to indicate briefly their principal modifi- 

 cations. When the axis of the ovule remains straight, as it is always at 

 first, so that the micropyle is situated at the summit of the ovule, and the 

 chalaza coincides with the hilum, both lying at the extremity of the ovule 

 opposite the micropyle, the ovule is called orthotropous or atropous (gerad- 

 laufig). When the ovule curves over on the end of the funiculus, so that 

 the upper part of the latter comes to lie parallel with one side of the 

 ovule and grows together with it, the ovule is named miatropous (gegenMu- 

 J-ig), In an ovule of this kind the chalaza Hes at the geometrical summit of 



IT 



