124 



MORPHOLOGY OF ANGIOSPERMS 



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 Asclepias and in 

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



Fig. 55.— Podophyllum peltatum. Mitosis in pollen mother-cell. A, telophase of first 

 division ; 2?, late anaphase of second division ; G, 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 Mother. 26 



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 Asclepias seems to have been made 

 bv Stevens 41 in 1898 ; and the fourth independent discovery 

 of it was by Gager 5S in 1902. Elving, 7 Wille, 15 and Stras- 

 burger 12 showed 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 Carex 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 : '~ 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 



