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INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



the time of development of the two spindle poles, the wallward or 

 generative pole developing more slowly than the vegetative, pre- 

 sumably because of the smaller amount of cytoplasm associated 

 with the former. With the onset of the anaphase, the asymmetry 

 becomes less pronounced. In the telophase the generative chromo- 

 somes are arranged in a plane surface parallel to the inner wall of 

 the microspore, while the vegetative ones form a somewhat hemi- 

 spherical pattern. 



Symmetrical spindles have been observed only occasionally. To 

 mention a few examples, in Asclepias (Gager, 1902) and Anthericum 

 (Geitler, 1935) both the poles of the spindle are blunt; in Adoxa 

 (Lagerberg, 1909) both are more or less pointed; and in Podophyllum 



Fig. 96. Allium cermium, first division of microspore, 

 phase. C, end of anaphase. {After Brumfield, 1941.) 



meta- 



(Darlington, 1936) both symmetrical and asymmetrical spindles are 

 said to occur in the same loculus. Further, in Adoxa (Lagerberg, 

 1909), Myricaria (Frisendahl, 1912), Sambucus (Schurhoff, 1921), 

 Colylanthera (Oehler, 1927), and Uvularia (Geitler, 1935) the spindle 

 is not situated near the wall of the pollen grain but occupies almost 

 the entire width of the latter. In any case the cells formed by the 

 division are always unequal, although the conditions which bring 

 about this result are not clearly understood. In Cuscuta (Fedort- 

 schuk, 1931) and Slrychnos (Mohrbutter, 1936), where the daughter 

 cells are sometimes of the same size, this is clearly an abnormality 

 leading to the formation of double microspores, each of which is des- 

 tined to divide again to give rise to the vegetative and generative cells 

 (Fig. 99F). Double pollen grains comprising two units, each with its 

 own generative and vegetative cells, have also been figured in Podos- 

 temon subulalus (Magnus, 1913) (Fig. 9QH, I). The separating wall 

 between the two pollen grains is pitted (Fig. 99/) and only one of them 



