124 



MORPHOLOGY OF AXGIOSPERMS 



cells of Cynanchum, but seems not to have noted the formation 

 of a tetrad; but the tetrad, consisting of a row of four micro- 

 spores, and referred to above as discovered by Strasburger and 

 by Frye in 1901 in a number of species of Asdepias and in 

 Cynanchum, was so unusual as to disguise its tetrad nature, and 



A B C 



TPiQ.tt.PodophyllumpeUatum. Mitosis in pollen mother-cell. A, telophase of first 

 division ; , late anaphase of second division ; (7, telophase of second division ; the 

 nuclei of the four microspores are formed, but the cell walls, as is characteristic of 

 simultaneous division, have not yet appeared. After MoxTiEE. 29 



besides, the enlargement and consequent readjustment of the 

 spores soon break up the row (Fig. 58). The first record of 

 the occurrence of a tetrad in Asdepias seems to have been made 

 by Stevens 41 in 1898 ; and the fourth independent discovery 

 of it was by Gager 58 in 1902. Elving, 7 Wille, 15 and Stras- 

 burger 12 show r ed that in various species of the Cyperaceae a 

 tetrad is formed although only one microspore becomes func- 

 tional, the other soon disorganizing. Juel 50 has recently made 

 a thorough study of Car ex acuta (Fig. 59). He finds that the 

 two characteristic nuclear divisions take place, and that a 

 cell-plate is formed at each division. The cell-plates are soon 

 resorbed, however, so that the four nuclei lie free within the 

 wall of the mother-cell. Three of the nuclei then disintegrate, 

 while the fourth becomes the nucleus of the single functional 

 microspore; and the wall of the mother-cell, inclosing the four 

 nuclei, becomes the wall of the microspore. In Zostera marina 

 Rosenberg 57 has described the tetrad division of the remarkably 

 elongated mother-cell (Fig. 11). The divisions are longitudinal 

 and in parallel planes, resulting in four remarkable filiform 



