THE ANGIOSPERMAE 1477 



pollination therefore results in two kinds of offspring : purely matro- 

 clinous forms, which are pseudogamously produced from diploid oospheres, 

 and also true sexual hybrids. The latter can subsequently perpetuate them- 

 selves apomictically and form new taxonomic units. 



The great majority of apomicts produce their embryos in one w^ay or 

 another from diploid nuclei, /.^., parthenogenesis is accompanied by apo- 

 spory or diplospory. The occurrence of " true " parthenogenesis of a 

 haploid oosphere, at least under natural conditions, is doubtful and the 

 evidence rests largely on supposition. Direct observation has occasionally 

 shown divisions in unfertilized oospheres, but there is no evidence that 

 they proceed to form viable embryos. One of the most interesting experi- 

 ments in this connection was the pollination of isolated female flow^ers of 

 Humidus lupulus with pollen of Urtica and Cannabis. Embryos in young 

 seeds were produced, though there was no evidence of fertilization and no 

 endosperm was formed. The embryos, however, all died without germina- 

 tion. Solanum nigrum pollinated with S. luteiim pollen has, however, pro- 

 duced about 20 per cent, of viable haploid offspring and haploid plants have 

 also been raised in Datura and Nicotiana as a result of pollination with 

 "foreign" pollen. In the case of Solanum, pseudogamy could be proved. 

 Pseudogamous stimulation of haploid embryo development has also been 

 proved in several Orchids but it is not known if these embryos can survive. 



An apparently good case of haploid parthenogenesis has recently been 

 described by Lindstrom and Ross. A tomato plant with twelve univalent 

 chromosomes arose from a haploid oosphere. There was a high degree of 

 microspore abortion but self-pollination gave a few diploid seedlings, which 

 were fertile. Adventitious shoots arising from the callus of cuttings were 

 frequently diploid, apparently due to the refusion of dividing nuclei in 

 the callus cells. These shoots were, of course, completely homozygous. 



The origin of diploid embryo sacs is a matter of some interest. They 

 are formed either from a complete megaspore mother cell, or from a dyad 

 cell after one division of the mother cell. The absence of chromosome 

 reduction is accounted for by one of three distinct processes, (i) The first 

 division of the nucleus in the megaspore mother cell is a somatic mitosis. 

 (2) The first division resembles a first meiotic division (heterotypic) but 

 the chromosomes at anaphase come together again to form a single, diploid 

 restitution nucleus. (3) The first division is pseudo-homotypic with con- 

 tracted chromosomes but there is no bivalent formation, no chiasmata, and 

 the chromosomes split at metaphase. All three types may occur in the same 

 species if the mother cell is directly transformed into an embryo sac. If 

 the mother cell forms a dyad of cells of which one develops into an embryo 

 sac, then its division follows patterns (2) and (3) only. 



Apospory w^as first described by Rosenberg in Hieracium flagellare, in 

 which a somatic cell near the normal spore-tetrad enlarges into a diploid 

 embrvo sac, either along with or in substitution for a normal haploid sac 

 formed from a spore (Fig. 1354)- Numerous other cases are known in- 

 cluding the genera Malus, Crepis, Hypericum, Poa and Rammculus. In 



