APOMIXIS 329 



size of the cell and a vacuolation of its cytoplasm, both taking place 

 prior to nuclear division. 



Unclassified Cases. In several plants it is difficult to draw a 

 sharp distinction between generative and somatic apospory. Atra- 

 phaxis frutescens (Edman, 1931) may be cited as an example of this 

 kind. Here we have a multicellular archesporium. Usually only 

 one or two of its cells, those which have a more central position, 

 take the characters of megaspore mother cells, while the rest undergo 

 a series of mitotic divisions. The true mother cells or their deriva- 

 tives soon degenerate. The aposporic embryo sacs arise from either 

 of the following sources: (1) the derivatives of the potentially 

 sporogenous cells lying close to the megaspore mother cell and 

 greatly resembling it in appearance (Fig. 189), or (2) the purely 

 somatic cells of the chalaza (Fig. 190). 



In Antennaria alpina and some other species of this genus (Juel, 

 1900; Stebbins, 1932; Bergman, 1941) the megaspore mother cell 

 may divide meiotically or mitotically. In the first or meiotic type, 

 the chromosomes become greatly contracted and lie scattered over 

 the spindle in a disorderly fashion. The separation of the daughter 

 chromosomes takes place irregularly, so that although the number 

 is reduced it is not exactly halved and several univalents are left 

 out altogether. The next division may result in a tetrad of four 

 megaspores, but more often five, six, or even seven cells may be 

 formed. The gametophytes arising from them are functionless and 

 soon degenerate. In the second or mitotic type, the dividing cell 

 grows vigorously and attains a large size even before the onset of the 

 prophase. There is no reduction in the number of chromosomes and 

 the embryo sacs derived in this way are diploid and functional. 



In Alchemilla arvensis (Murbeck, 1901 ; Boos, 1924) the megaspore 

 mother cell enters a meiotic prophase, but the nucleus soon de- 

 generates. Meanwhile, the surrounding cells, which are potentially 

 sporogenous, undergo a number of mitotic divisions resulting in a 

 few parietal cells and two to six axial cells, which may divide either 



of degeneration; chalazal cell showing increase in size and vacuolation. C, mega- 

 spore tetrad and large nucellar cell destined to give rise to embryo sac. D, normal 

 and aposporic embryo sacs growing simultaneously. E, two fully developed embryo 

 sacs; lower is probably of aposporic origin. F-H, some stages in development of 

 aposporic embryo sac; note progressive degeneration of megaspore tetrad. (After 

 Rosenberg, 1907.) 



