61 KAXDA— VERBENA 1920] 



the pollen grains. The wall of each microspore gradually thickens 

 and sometimes a great many starch grains may be observed in the 

 interior (tig. 39). Cases of accumulation of starch grains in the 

 pollen have been reported by Murbeck (15), Ishikawa (ii), 

 and others. In Oenothera Ishikawa states that "the plasm con- 

 taining starch grains in the pollen tube is poured into the attacked 

 synergid," but in this case no starch is present in the pollen tube 

 (fig. 42). A large vacuole appears in the pollen grain for a time 

 (fig. 40) , but it soon fades away and the first vegetative cell is cut 

 off (fig. 41). More advanced stages could not be observed, as the 

 contents and wall of the pollen grains become extremely dark in 

 color. While these changes are occurring, the tapetum and middle 

 layer disintegrate. 



Fertilization 



It is very difficult to obtain clear pictures of the stages in which 

 the male nuclei are on the point of fusing with the egg cell and 

 the endosperm nucleus. In the first place the egg apparatus is 

 rendered very indistinct through the presence of deeply staining 

 cytoplasmic substances around it. I believe this deeply staining 

 material is the result of a concentration of the cytoplasm and the 

 inclusion within it of nutritive substances destined for the endo- 

 sperm. The abundance especially of starch grains around the 

 egg apparatus greatly confuses its appearance with the gentian 

 violet stain. Secondly, the synergids seem to be more ephemeral 

 in Verbena than in other plants, and soon become converted into a 

 tenacious mucus-like material. This material from the dis- 

 organized synergids also stains very deeply. Thirdly, when the 

 pollen tube enters the egg apparatus, a part of the disorganized 

 nucellar cap penetrates into it with the tube and always gives rise 

 to a figure of peculiar shape and staining properties (figs. 42-44, 

 46). Merrell states that in Silphium "the pollen tube passes 

 along the outside of the cap which usually crowns the embryo sac 

 and enters the sac just beyond its free margin." In Verbena, 

 however, the pollen tube, entering the sac at the micropylar end, 

 thrusts itself through the nucellar cap (fig. 42), just as in Lemna, 

 described by Caldwell. 



