THE MICROSPORANGIUM 



45 



in the Thallophytes and other lower plants, the simultaneous type 

 is the more ancient and the successive type the derived. In gen- 

 eral, the former is prevalent in the majority of dicotyledons and the 

 latter in the majority of monocotyledons. There is no hard and 

 fast rule, however, and exceptions are frequent. Thus the suc- 

 cessive type is found in a few dicotyledonous families like the 

 Asclepiadaceae, Podostemonaceae, and Apocynaceae, and the simul- 

 taneous type in a few monocotyledonous families, viz., the Iridaceae, 

 Taccaceae, Juncaceae, and Dioscoreaceae, and in several genera 

 of the Liliaceae, Palmaceae, and Orchidaceae. 



In Magnolia (Farr, 1918) there are isobilateral tetrads formed by 

 furrowing instead of by cell plates. A cleavage furrow starts after 



A B C D E 



Fig. 34. Cytokinesis in microspore mother cells of Zea mays. A, anaphase of 



Meiosis I. B,C, laying down of partition wall after Meiosis I. D, telophase of 

 Meiosis II. E, isobilateral tetrad. {After Reeves, 1928.) 



Meiosis I, but its development is arrested during the second meiotic 

 division. It resumes growth at the end of Meiosis II and forms a 

 partition through the equatorial region of the mother cell. At the 

 same time additional furrows originate at the periphery, and the 

 two dyad cells now become subdivided to give rise to the four 

 microspores. A similar condition occurs in Anona (Juliano, 1935a) 

 and Asimina (Locke, 1936). 



The Microspore Tetrad. As mentioned above, the microspores 

 are usually arranged in a tetrahedral (Fig. 35 A) or isobilateral (Fig. 

 35B) fashion, but there are exceptions (Fig. 35C-E). A decussate 

 arrangement of the cells has been recorded in Magnolia (Farr, 1918), 

 Atriplex (Billings, 1934), Comas (D'Amato, 1946), and many other 

 plants. In some genera of the Asclepiadaceae (Gager, 1902) and 

 in the genus Halophila of the Hydrocharitaceae (Kausik and Rao, 

 1942) the mother cells divide transversely so as to give rise to 



