32 BOTANY. 



The forameu of the ovule is also called micropyle. The nucleus and 

 integument arc united at the base of the ovule by a cellulo-vascular 

 membrane, called cJialaza. The hilum indicates the oi-ganic base of the 

 ovule, the foramen marking the apex. The primine, secundine, and nucleus, 

 are ahvays united together at some point of their surface. When this union 

 takes place at the base of the ovule, as in its embryonic condition, this is said 

 to be orlhotropal, or at r opal. When the ovule is curved downwards, so as 

 to approach the placenta, it is camptotropal ; when curved downwards, and 

 grown to the lower half, anatropal ; when attached by the middle, so that 

 the foramen is at one end, and the base at the other, it is cainpylotropal^ or 

 amphitrojyal ; when shaped like a horse-shoe, lycotropal ; when anatropal, 

 with the raphe half loose, semianatropal. By raphe is meant the vascular 

 connexion between the base of the ovule and the base of the nucleus, in 

 cases -where these two bases do not coincide as they do in the orthotropal 

 ovules. 



An ovule is said to be ascending when attached to a parietal placenta, with 

 the apex directed upwards. It may hang from an apicilar placenta at the 

 summit of the ovary, and be inverted or pendulous ; or it may be suspended 

 from a parietal placenta near the summit. When two ovules in the same 

 cell are placed side by side, they are collateral, and their relative positions 

 may otherwise vary. 



c. Fertilization. 



The fertilization of a flower usually results from the action cf pollen upon 

 the stigma, which in some manner causes the development of an embryo 

 within the nucleus. Authors disagree as to the precise manner in which this 

 action is exerted. The theory most generally adopted is, that the pollen 

 grains falling on the stigma are detained there, and soon exhibit a protrusion 

 of the inner coat, or intine, in the shape of a tube, which penetrates the 

 stigmas, and passes down through the style, ultimately to reach the embryo. 

 The result of this action is the formation of a vital point (a single cell), which 

 ultimately becomes the embryo, and from which a new plant may be produced 

 by exposure to the proper conditions. Sometimes more than one embryo may 

 be developed in the same ovule. The embryo derives the material of its 

 growth from the surrounding tissues, and the Avhole series of phenomena is 

 attended by the evolution of heat, which sometimes is quite conspicuous. 

 Authorities disagree as to whether or not the germinal vesicle exists in the 

 embryo-sac before the application of the fovilla. In some cryptogamous 

 plants the vital spores are discharged from their envelope without any 

 apparent union of cells of two different sexual characters : in the Confervas 

 and Diatomaceae, however, there is a union of the contents of two different 

 cells, by means of tubes, which are protruded from one into the other. This 

 process, called confugatioyi., results in the production of germinating bodies. 

 When the pollen of one species of plant fertilizes the ovule of another 

 species, the result is a hybrid. These, however, are of rare occurrence in 

 nature. 

 32 



