Apomixis maybe defined (Winkler, 1908, 1934) as the substitution for sexual reproduction (amphimixis) of an asexual process which does not involve any nuclear fusion. For the sake of convenience it may be subdivided into four classes.
- Non-recurrent apomixis
- Recurrent apomixis
- Adventive embryony
- Vegetative propagation
In the first, or non-recurrent apomixis, the megaspore mother cell undergoes the usual meiotic divisions and a haploid embryo sac is formed. The new embryo may then arise either from the egg (haploid parthenogenesis) or from some other cell of the gametophyte (haploid apogamy). Since the plants produced by this method contain only a single set of chromosomes, they are usually sterile, and the process is not repeated from one generation to another.
Haploid embryos may result in various ways. The first and most important source is the unfertilized egg. Failure of fertilization may be due to any of the following causes: (1) absence of a pollen tube, (2) inability of the tube to discharge its contents, (3) an insufficient attraction between the male and female nuclei, (4) an early degeneration of the sperms, and (5) a discordance in the time of maturation of the egg and the entrance of the male gametes.
In the pollen of S. luteum readily germinated on the stigmas of S. nigrum and the pollen tubes reached the embryo sacs in the normal way. One of the sperms also entered the egg but failed to fuse with the female nucleus and soon degenerated. Its presence sufficed, however, to stimulate the haploid egg to develop parthenogenetically and give rise to an embryo.
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In the second or recurrent type of apomixis, the embryo sac may arise either from a cell of the archesporium (generative apospory) or from some other part of the nucellus (somatic apospory). There is no reduction in the number of chromosomes, and all the nuclei of the embryo sac are diploid. The embryo may arise either from the egg (diploid parthenogenesis) or from some other cell of the gametophyte (diploid apogamy).
Holmgren (1919) has made a very detailed study of the embryology of several species of the genus Eupatorium.
Some species are entirely normal and reproduce sexually, but E. glandulosum, which is a triploid, is apomictic. Here the division of the megaspore mother cell differs little from a somatic mitosis except in the fact that the cell elongates considerably, demolishing the nucellar epidermis and becoming vacuolate even before it is ready to divide. There is no synapsis or pairing of chromosomes. Three nuclear divisions take place, to give rise to an eightnucleate embryo sac with two or three antipodal cells which may undergo a further division with or without the formation of a cell wall. The embryo arises from the unfertilized but diploid egg cell.
Rosenberg (1907) described the occurrence of somatic apospory in three species of the genus Hieracium (subgenus Pilosella), viz., H. excellens, H.flagellare, and H. aurantiacum. The megaspore mother cell goes through the usual meiotic divisions, but at just about this stage a somatic cell situated in the chalazal region begins to enlarge and becomes vacuolated. This cell gradually increases in volume, encroaching upon the megaspores and finally crushing them. The aposporic embryo sac, arising from it, has the unreduced chromosome number and is able to function without fertilization.
In the third type, whatever the method by which the embryo sac is formed and whether it is haploid or diploid, the embryos do not arise from the cells of the gametophyte but from those of the nucellus or the integument. This is called adventive embryony or sporophytic budding.
Here we have no alternation of generations, as the diploid tissues of the parent sporophyte directly give rise to the new embryo.
In the fourth type the flowers are replaced by bulbils or other vegetative propagules which frequently germinate while still on the plant.