1476 A TEXTBOOK OF THEORETICAL BOTANY 



embryo sac. This is really one form of vegetative reproduction and 

 is closely linked to Vivipary, in which embryonic plants arise 

 vegetatively from floral parts, often in replacement of the whole or 

 part of the flower. 

 Pseudogamy. Certain apomictic plants {e.g., species of Riihus) require 

 pollination to enable them to produce embryos, although no fertiliza- 

 tion takes place. 



There are thus three ways in which agamospermy may occur: diplo- 

 spory followed by diplo-parthenogenesis or apogamy; apospory followed 

 by parthenogenesis or apogamy; and adventitious embryony. 



The prevalence of apomixis, especially in certain genera, raises numerous 

 points of theoretical importance, some of which we shall discuss in Volume 

 III under Genetics. One of the first to occur is the question whether a 

 diploid oosphere is to be reckoned as sporophytic or gametophytic, or, in 

 other words, whether the haploid condition is the essential feature of a 

 gametophyte. If the diploid oosphere is simply a special type of somatic 

 cell, then its ofl^spring should be the same as those arising from adventitious 

 embryony, of which it is simply a special case. Winkler has shown that 

 this is not so. The offspring of true adventitious embryos always exactly 

 resemble the female parent but in dioecious species such as Thalictrum 

 fendleri, the offspring of diploid oospheres comprise both male and female 

 plants, while in Bryonia dioica, the offspring may be wholly male, as in 

 parthenogenetic animals. Further, adventitious embryony is almost 

 always accompanied by polyembryony, parthenogenesis never, 



Strasburger showed that apomictic genera are characterized by high 

 chromosome numbers, or as would now be said, they are polyploids. They 

 also, as Ernst showed, behave in many ways like hybrids, although hybri- 

 dity, apart from polyploidy, is not itself a cause of apomixis. In this con- 

 nection the taxonomic aspects of apomixis are of the highest interest, 

 especially from the point of view of the origin of species. It has long been 

 known that some genera are taxonomically divisible into a great many units, 

 differing only in minor characters but showing a high degree of constancy. 

 These are called "micro-species", and they form polymorphic species 

 complexes. At the beginning of the century it was shown by Juel and 

 Murbeck that two of the species complexes, those of Taraxacum and Hiera- 

 cium, were characterized by apomixis and investigation has shown that this 

 is also true oi other genera in which such species complexes exist. A number 

 of important European genera are in this category : the two mentioned 

 above and Potentilla, Alcheniilla, Sorbtis, Crepis, Riibus, Rosa, Poa and 

 Calamagrostis, as well as others, while many more no doubt exist in other 

 floras, e.g., Elatostema. 



In Riihus, as in some other genera, there are a few, original, diploid 

 species which are fully sexual, but the greater number of the "species" 

 are polyploid and apomictic. As in Hieraciiim, however, the apomictic 

 species also produce some haploid, fertilizable embryo sacs. Cross- 



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