Forms of Ovules 



123 



There are three forms of ovules, differing in the arrange- 

 ment of their parts : — 



Orthotropous ovule (fig. 236, a). In this case the nucleus 

 is straight, the chalaza and hilum are in proximity, and the 

 micropyle is removed to a distance from the funiculus. This 

 form is rare, being met with in the Rhubarb and its allies in 

 the natural order Polygonaceae. 



Anatropous ovule (fig. 236, b). In this case the nucleus 

 is straight, but is inverted upon the funiculus, so that the micro- 

 pyle is brought down into close proximity with the stalk, whilst 

 the hilum and chalaza are separated, being united by a pro- 

 longation of the funiculus known as the raphe. This form is 

 well seen in the Dandelion and White Water-lily. 



Fig. 236. —Diagrammatic longitudinal pections of ovules : A^ orthotropous or straight ; 

 B^ anatropous or inverted ; and C, campylotropous or bent ovule, ai, outer integu- 

 ment or primine ; ii, inner integument or secundine ; wz, the micropyle ; k, the nucleus, 

 with the embryo-sac, em ; c, the chalaza or base of the nucleus ; ^ the funiculus ; r, 

 the raphe of the anatropal ovule, formed by the fusion of the outer integument and 

 funiculus. (After Prantl.) 



Campylotropous or bent ovule (fig. 236, c). In this case 

 the nucleus is bent, so that whilst the micropyle is brought near 

 the funiculus, as in the anatropous form, the chalaza and hilum 

 are in close proximity as in the orthotropous form, and there is 

 no need of a raphe. The Wallflower and Mallow are examples 

 of this kind. These three forms are united by many modifi- 

 cations. 



Thalamus and Disc. — The thalamus, or end of the flower- 

 stalk upon which the various floral organs are inserted, is often 

 specially modified. Thus in all plants with a free central 



