THE MALE GAMETOPHYTE 



123 



Zostera, Cyperaceae, and Asclepias the claim -was disproved 

 long ago ; and even these have now been cleared up, so that no 

 case is known in which a pollen mother-cell becomes a micro- 

 spore directly without the tetrad divisions. It does not seem 

 improbable that such a case may exist, for cases of oogenesis 



D 



G H * 



Fig. 54.— Frit ill 'aria persica. Sections showing the two nuclear divisions by which four 

 microspores are formed in the mother-cell by the successive (bilateral) method; 

 x 530. A, very young mother-cell; B, nucleus in synapsis; C\ 12 chromosomes, 

 one of them rather indistinct, within the nucleus; I>, mitotic figure of the first 

 division showing the short, thick chromosomes characteristic of the reduction 

 division; E, later stage of first division, showing vertical view of the 12 chromo- 

 somes; F. side view of same stage showing 12 chromosomes passing to the upper 

 pole, only 10 for the lower pole being in sight; G, formation of wall between 

 daughter nuclei ; II, second division ; /, formation of walls. — After Stkasburger.™ 



like that of Lilium are not rare, where the mother-cell gives 

 rise directly to a single megaspore. As stated, in 1886 Wille 15 

 found no tetrad in Asclepias syriaca; and in 1892 Chau- 

 veaud 20 observed the reduction division of the pollen mother- 



