APOMIXIS 



435 



parthenogenesis in which fertihsation fails first, and a type in which 

 meiosis fails first. The effect will be that, in the first, both 

 generations (normally haploid and diploid) will be haploid, when the 

 reproduction is called haploid parthenogenesis, and in the second, 

 both generations will be diploid, when the reproduction is called 

 diploid parthenogenesis (Fig. 104) . It will be seen that this distinction 

 can only be made with certainty where the origin of the new system 

 is known or the normal type of reproduction can be compared with 

 the aberrant ; this, however, is usually possible. Further, in such 

 a case as that of apparent diploid parthenogenesis in Chara crinita 

 (Ernst, 1918) it is possible, although not plausible, to assume that 

 failure of a mitotic division, not failure of meiosis, was the starting- 

 point of a parthenogenetic mode of reproduction (cf. Winkler, 1921). 



In the higher animals the specialised germ-ceU is always the 

 immediate product of meiosis, or some modification of meiosis, in the 

 specialised mother-cell, and where meiosis fails in parthenogenesis 

 the specialised function of the mother-cell (oocyte) and the gamete 

 (egg-cell) are always retained. But in plants with an alternation of 

 generations there are specialised spore mother-cells and specialised 

 gametes developed at different parts of the life cycle, and un- 

 specialised cells may be substituted for either or both of these when 

 meiosis and fertilisation are omitted. Thus not only the character- 

 istic processes but also the characteristic organs of sexuality are lost. 

 Where the spore mother-cell either fails to develop or does not 

 function in reproduction we speak of apospory, and where the normal 

 gamete cell does not function we speak of apogamy. When both of 

 these are combined only the external character of seed production 

 distinguishes the development of the embryo from vegetative 

 propagation. 



Apomixis will first be considered in relation to the life cycle in the 

 most important groups in which it occurs. 



2. APOMIXIS IN RELATION TO THE LIFE CYCLE 



(i) Higher Animals (Metazoa). (a) Diploid Parthenogenesis. 

 In many species of insects, crustaceans and nematodes this form 

 of parthenogenesis is obligatory. Parthenogenetic females produce 

 only their like, by a failure of meiosis at maturation in the oocyte. 



