46 



INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



linear tetrads (Fig. 36A-D). T-shaped tetrads also occur some- 

 times as in Aristolochia (Samuelsson, 1914) and Butomopsis (Johri, 

 1936). In Zostera (Rosenberg, 1901) the elongated microspore 

 mother cells, measuring 5 by 60 microns at the time of meiosis, 

 divide in a plane parallel to the longitudinal axis of the cell, result- 

 ing in a group of four filiform cells which undergo further elongation 

 and become approximately 2000 microns long when mature. 10 Of 

 considerable interest are Musa (Juliano and Alcala, 1933), Neottia 

 (Goebel, 1933), Agave (Vignoli, 1936, 1937), Nicolaia (Boehm, 1931), 

 Habenaria (Swamy, 1946), Laurus (Battaglia, 1947), and Ottelia 

 (Islam, 1950) in which two or three types of dispositions may be 

 found in one and the same species. 



Occasionally there are either fewer than four spores resulting from 

 the divisions of the microspore mother cell, or more than four. 



A 'B " — ^C ^-^D ^^E 



Fig. 35. Diagram showing different types of microspore tetrads. A, tetrahedral. 

 B, isobilateral. C, decussate. Z),T-shaped. ^linear. (B-E, ajter\ Boehm, 1931.) 



The former condition originates as the result of a failure of one 

 division, or the formation of a "restitution nucleus" after the first 

 division, or an irregular wall formation giving rise to one binucleate 

 and two uninucleate spores. The latter condition, i.e., the forma- 

 tion of more than four spores (polyspory), usually results from the 

 occurrence of lagging chromosomes which organize into micronuclei. 

 In general, however, such abnormalities in the number of micro- 

 spores are found only in hybrids characterized by a high degree of 

 sterility and the pollen grains arising in this way are nonfunctional. 

 Usually the microspores soon separate from one another but in 

 some plants they adhere in tetrads to form the so-called "com- 

 pound" pollen grains. 11 As examples may be cited Drimys, Anona, 



10 Filiform pollen grains also occur in Phyllospadix and Cymodocea, but the 

 method by which they arise does not seem to have been studied so far. 



11 For detailed information on such variations of external form, see Wodehouse 

 (1936) and Erdtman (1943, 1945). 



