282 THE FLOWER. 



530. Origin and Nature of the Ovule. It has been already 

 stated in general terms that ovules are peculiar outgrowths 

 or productions, generally of the margins of carpellary leaves 

 (515) ; that they are composed of parenchymatous cellular sub- 

 stance, at least as to the nucleus, of which the simplest ovule 

 wholly consists (526) ; that the coats originate subsequent!}- to 

 the nucleus ; and that the outer coat is of later origin than the 

 inner one. (518.) The mamiliform protuberance of which the 

 forming ovule at first consists originates in one or more cells of 

 a layer directly beneath the epidermis. 1 



531. The morphological nature of the ovule has been much 

 discussed. The commonly prevalent view was that the ovule 

 is homologous with a leaf-bud, and that its nature is in some 

 degree illustrated by such buds as those which develop on the 

 margins of the leaves of Bryophyllum, as shown in Fig. 322. 

 But such buds, and the bulblets or fleshy buds which appear on 

 the face of certain leaves, follow the universal order of budding 

 growth, that is, are centripetal in development, the outermost 

 parts being the earlier and the inmost the later formed. The 

 ovule, on the contrary, is basipetal or centrifugal in develop- 

 ment, the nucleus being first and the outer coat last formed ; 

 therefore the coats are not homologous with sheathing leaves, 

 nor the nucleus with a vegetative axis. The older theory has 

 accordingly given way to the present one, in which the ovule 

 answers to the lobe of a leaf peculiarly transformed, or to an 

 outgrowth of a leaf, whether from its edges or surface. The 



apotropoas, epitropous, and heterotropous (the first two new, the last employed 

 in a new sense), the more so since the application is confused with hypo- 

 thetical considerations and the necessity of bringing the ovules ideally back 

 to ascending or horizontal positions. It may be stated, briefly, that Hetero- 

 tropous, in Agardh's terminology, applies to the normal position of collat- 

 eral ovules, with rhaphes back to back, in opposite directions on the two 

 halves of the placenta ; Apotropous, to an erect or ascending ovule with its 

 rhaphe next the placenta! axis, and a hanging one has its rhaphe averse 

 from it ; Epitropous, when an erect or ascending ovule has its rhaphe averse, 

 and a hanging one has it adverse. 



1 Hofmeister's statement that the simple ovule of Orchis originates in 

 the division of a single epidermal cell (and is therefore a trichome) is con- 

 troverted by Strasburger and by Warming. The latter adds the remark, 

 that even if it were so in cases of extreme simplicity, this would not invali- 

 date the proposition that the ovule is to be regarded as the homologue of the 

 lobe of a leaf. Such a lobe is not rarely reduced to a single bristle. For 

 the whole subject of the origin, development, morphology, and theory of the 

 ovule, see Warming's very elaborate and perspicuous memoir, De 1'Ovule ; 

 also the papers of Celakowsky, Van Tieghem, &c., referred to in notes to 

 paragraphs 500, 601. 



