40 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



ity the partitions between the sporangia break down and the loculi 

 become continuous. 



The genus Arceuthobium is unique in having a single annular 

 pollen sac forming a continuous ring around a central column of 

 sterile cells called "columella" (Stadtler, 1923; Fisek, 1924; Thoday 

 and Johnson, 1930; Dowding, 1931). Regarding the origin of this 

 condition there is, however, some difference of opinion. Stadtler 

 (1923) thinks that the anther is at first multilocular but the par- 

 titions break down at maturity. Pisek (1924), on the other hand, 

 contends that it is unilocular from the commencement, and this is 

 supported by Thoday and Johnson (1930) who state that even in 

 the youngest anthers there is a ring-shaped archesporium surround- 

 ing the central columella. Dowding (1931) agrees regarding the 

 continuity of the archesporium but finds that the columella exhibits 

 a considerable amount of variation. It frequently forms a sort of 

 flange dividing the anther into two halves; sometimes the first 

 flange tends to disappear, and a new one arises at right angles to it. 

 Rarely, the flanges give out branches extending outwards to the 

 anther wall. In Dowding 's opinion these flanges of the columella 

 are to be regarded as remnants of the septa which once separated 

 four distinct archesporia. 



Although all the sporogenous cells in the anther are potentially 

 capable of giving rise to microspores, some of them frequently de- 

 generate and become absorbed by the remaining cells. In Ophiopo- 

 gon (Maheshwari, 1934) and Holoptelea (Capoor, 19376) some of the 

 sporogenous cells do not reach even the mother cell stage and prob- 

 ably serve to nourish the remaining cells. In Zoster a (Rosenberg, 

 1901) most of the sporogenous cells divide longitudinally to form the 

 numerous long microspore mother cells but others interspersed be- 

 tween them undergo transverse divisions and give rise to sterile 

 cells which are later crushed and used up by the functioning cells. 

 In certain members of the Gentianaceae (Guerin, 1926) which are 

 devoid of any well-formed tapetum, the nutritive function is taken 

 over by some of the sporogenous cells themselves. These become 

 sterile and do not go through the reduction divisions (Fig. 32). 

 In Kigelia (Venkatasubban, 1945) degeneration takes place at a 

 later stage; some of the microspores in a tretrad fail to develop fur- 

 ther and become functionless. 



Cytomixis. While making a study of Oenothera gigas and 0. 



