578 BOTANY PART n 



or induced by external conditions. It occasionally alternates with 

 sexual reproduction, especially in lower plants in which the reduction 

 division follows on the union of the gametes, and results from the 

 influence of external conditions on the gametes. 



APOGAMY, on the other hand, is the obligate apomictic formation 

 of an embryo from cells of a diploid or heteroploid gametophyte. 

 Ovogenic apogamy is when the young plant arises without a sexual 

 process from the egg-cell. Somatic apogamy is when it arises from 

 other cells of the gametophyte. APOSPORY is the complete omission 

 of spore formation. 



Thus in Fig. 561 a case of the apomictic development of adventitious 

 embryos is represented. Vegetative growths from synergidae or 

 from adjoining cells of the nucellus form in the embryo-sac and affect 

 or completely prevent the development of the fertilised egg. Nucellar 



FIG. 5(51. Vegetative formation of embryos in Funkia ovata. n, Nucellus with cells in process 

 of forming the rudiments (ae) of the adventitious embryos ; S, synergidae ; E, egg-cells, in 

 the figure on the right developing into an embryo ; n, inner integument. (After STRASBURGER.) 



embryos of this kind are formed only after the stimulus of pollination 

 in Funkia and Citrus aurantium. In the well-known Euphorbiaceous 

 plant Caelebogyne ilicifolia, which occurs in cultivation in female 

 specimens only, and in species of Calycanthus, it takes place without 

 this stimulus. In these two latter cases we have complete loss of 

 sexuality and somatic apogamy. Numerous cases of ovogenic 

 apogamy have been discovered of recent years. In Alchemilla, 

 Thalidrum, Taraxacum, etc., the pollen grains are usually functionless 

 and the reduction of chromosomes in the development of the embryo 

 sac is suppressed, so that the nuclei retain the diploid number of 

 chromosomes ; the plants have become apogamous. According to the 

 investigations of OSTENFELD and ROSENBERG, the genus Hieracium 

 is of special interest, since the formation of the embryo within the ovule 

 may commence in very various ways. In most cases a tetrad formation 

 accompanied by a reduction division takes place, but only some of 

 these ovules are found to have a normal embryo -sac capable of 

 fertilisation; as a rule this is displaced by a vegetative cell which 

 develops into an embryo-sac aposporously (Fig. 562). In exceptional 

 cases apogamous embryo-sacs are formed. 



