146 



FUNDAMENTALS OF CYTOLOGY 



genesis, in which an unfertihzed egg develops with the diploid chromosome 

 number (Fig. 106). This is probably the commonest mode of apomixis 

 involving seed formation, being known to occur in a number of genera 

 in nature. The female gametophyte and egg in this case are diploid 

 because of a failure of meiosis at sporogenesis in the ovule, or as a result 

 of the development of the gametophyte directly from a somatic cell of 

 the ovule without spore formation (apospory). In such cases there is an 

 alternation of gametophytic and sporophytic generations in the life 

 cycle, but no alternation of chromosome numbers. (3) Apogamy, in 

 which an embryo arises from a gametophytic cell other than the egg. 

 This has occasionally been observed but is not known to occur regularly 

 in nature. (4) Adventitious emhryony, in which 

 a sporophytic cell of the nucellus or the integu- 

 ment buds into the embryo sac and produces an 

 embryo (Fig. 106). This ma}^ occur without the 

 stimulus of pollination (Euphorbia) or only after 

 syngamy has taken place (orange). (5) Polyem- 

 hryony, in which more than one embryo develops 

 in an embryo sac. One of these may develop 

 sexually in the usual manner, while others arise 

 in one of the ways mentioned above. Another 

 rare mode is the formation of several embryos by 

 proliferation from a single one at an early stage. 

 The following phenomena not included under 

 the heading of apomixis are also encountered at 

 times. (1) Gynogenesis, in which the male nucleus 

 enters the egg but then disintegrates, leaving the 

 egg to develop into an embryo with the maternal 

 nucleus only. (2) Androgenesis, in which the 

 maternal nucleus presumably disintegrates, for a monoploid plant 

 develops with paternal characters. (3) Parthcnocarpy , in which fruit 

 development takes place without the egg having been fertilized and 

 in different cases with or without the stimulus of pollination. Parthcno- 

 carpy occurs naturally in the banana and some strains of grapes and citrus 

 fruits, and it has been found possible to induce it with a growth hormone 

 in tomatoes, pepper, and tobacco. (4) Metaxenia, in which the embryo 

 or endosperm developed after a cross produces a visible modification in 

 the character of the maternal parent tissue enclosing it. This has been 

 observed in apples and the date palm. A visible effect of the male 

 parent on the character of the endosperm {xenia) is expected after some 

 crosses, for the male parent contributes nuclear material to the tissue, 

 but effects upon tissues where no such material is present must receive 

 another explanation. 



Fig. 106. — Embryo 

 sac of Alchemilla with one 

 embryo developing by 

 unreduced parthenogene- 

 sis (below) and another 

 by adventitious budding 

 (above). {After S. Mur- 

 heck.) 



