THE MEGASPORANGIUM 



75 



The genus Musa is of special interest, for here tetrads of four dif- 

 ferent kinds — linear, T-shaped, 1-shaped and isobilateral — may oc- 

 cur in the same species (Dodds, 1945). Tetrads of very variable 

 appearances have also been described in Poa alpina (Hakansson, 

 1943). 



Frequently a row of only three cells is seen in place of the usual 

 four. This is due to an omission of the second meiotic division in 

 one of the two dyad cells, usually the upper. 9 All intermediate 

 stages leading towards this condition have been seen. In some 

 plants the division in the upper dyad cell merely lags behind that 



D 



Fig. 52. Megaspore tetrads in Urginea indica. A, linear tetrad. 5,T-shaped 

 tetrad. C, tetrad showing decussate arrangement of megaspores. D, two tetrads 

 lying in same row. (After Capoor, 1937.) 



in the lower dyad cell, and all four cells are formed as usual; in 

 others the nucleus divides normally, but a separating wall is not 

 laid down; in still others the division is abortive and merely gives 

 rise to two degenerating clumps of chromosomes; and in a few 

 cases the nucleus degenerates without undergoing any division. 



Functioning Megaspore. Normally it is the chalazal megaspore 

 of the tetrad which functions and gives rise to the embryo sac, while 

 the remaining three megaspores degenerate and disappear. But 



9 It is to be noted that sometimes one gets a false impression of the occurrence 

 of a row of three cells either because of the plane of the section or because of the 

 orientation of the wall between the two upper megaspores so that one of the cells 

 lies superposed over the other. In such cases more careful focusing, or a study of the 

 adjacent section, reveals the presence of the fourth megaspore (see Graves, 1908). 



