Genetic Systems II | 199 



vidual flowers, which are modified spore-producing branches, de- 

 velop into specialized vegetative branches that fall from the plant, 

 subsequently to root and grow. Clone formation is the result. 



Besides these relatively simple modes of apomictic reproduction, 

 there are sorts involving seed production whose complexity is amaz- 

 ing and whose effects are greatly puzzling. They are known collec- 

 tively as agamospermy. Considering only the higher plants as exam- 

 ples, one must first remember that their life cycle has a gametophytic 

 period which separates meiosis and fertihzation and which alternates 

 with the sporophytic generation. The gametophyte may consist of 

 only a few cells (e.g., eight in many flowering plants), or it may be 

 a relatively massive multicellular plant, physiologically independent 

 of the sporcphyte ( as in most ferns ) . In either event, apomixis may 

 occur in this generation if the egg develops without fertilization. The 

 four basic types of apomixis in which the gametophytic generation is 

 involved are known collectively as gametophytic apomixis; this in- 

 cludes forms in which cells other than the egg may develop to pro- 

 duce a new sporophyte. Development of the egg without fertiliza- 

 tion is called parthenogenesis, while the term apogamety refers to 

 the production of a sporophyte from other cells of the gametophyte. 

 Ordinarily the products of either type of development are diploid; 

 haploid parthenogenesis and haploid apogamety are not of evolu- 

 tionary significance in vascular plants. 



Some upset of meiosis in the preceding sporophytic generation 

 has taken place, or meiosis may have been eliminated altogether. 

 Thus the resulting spores, and the gametophytes they produce, are 

 diploid. This may occur in two ways. The cells of the sporophyte 

 that would ordinarily undergo meiosis (e.g., in the ovule) may 

 undergo mitotic or only partly meiotic divisions; this is known as 

 diplospory. In some plants somatic cells of the reproductive struc- 

 tures may, through mitotic divisions, produce a gametophyte. This 

 process, apospory, likewise results in a diploid gametophyte. The 

 further development of such a gametophyte may involve partheno- 

 genesis or apogamety, as seen above. 



Development may be dependent upon the influence of a pollen 

 tube or on nutritive tissue (itself a product of fertilization), called 

 endosperm, that surrounds the apomictically produced embryo spo- 

 rophyte. Thus pollination is necessary, but fertilization of the egg 

 does not take place. This is called pseudogamy and is generally one 

 of the most difficult forms of apomixis to detect. 



Apomixis in the higher plants may progress to the point that the 

 gametophytic generation is suppressed completely. A variety of 

 agamospermy known as adventitious emhnjony results. Here cells 



