PLATE D 



I. — Nearly mature pollen grain. The central body is the vegetative nucleus 

 while the other is the generative cell. X 2,000. 



2. — Mature pollen grain. The extine is not shown. The killing fluid causes the 

 dry folded grains to become spherical. X 2,000. 



3. 4> 5. 7- — Various types of degenerate microspores from anthers bearing 

 microspores of the stage shown in Plate C, figure 6. X 2,000. 



6. — An aborting microspore from an anther containing half-grown microspores. 

 X 2,000. 



8. — An aborting microspore of the same type as that shown in figure 6 from an 

 anther containing nearly full-grown microspores as in Plate C, figure 8. As ia Plate 

 D, figiu-e 6, the cytoplasm and nucleus still appear normal. X 2,000. 



9, II. — Microspores of the same types and same age as figures 6 and 8, in which 

 degeneration has proceeded farther X 2,000. 



10. — An aborted microspore from an anther containing microspores of the stage 

 shown in Plate C, figure 8. Apparently abortion took place shortly following libera- 

 tion from the tetrad. 



12. — An early stage of degeneration in a full-grown i-nucleate microspore. X 2,000. 



13. — An early stage of degeneration in a full-grown i-nucleate microspore. A 

 ntmiber of normal microspores in this anther are already dividing. 



14. — An aborting microspore containing an. abnormally small amoimt of light 

 staining cytoplasm; from an anther containing i- and 2-nucleate microspores. X 2 ,000. 



15. — An aborted microspore from an anther containing i- and 2-nucleate micro- 

 spores. Apparently this is a late stage of the type of degeneration shown in figures 

 6 and 8. X 2,000. 



16. — An aborted microspore containing very scant cytoplasm. The nucleus has 

 completely degenerated and degeneration of the cytoplasm has begxm. 



