430 iminxesota hotaxical studies 



Pollen and Pollen Tubes 



A cross-section of an anther shows four pollen sacs, each having 

 an epidermal layer, an endothecium, one middle layer, and a tapetal 

 layer. The tapetal cells, which have from one to three nuclei in 

 each, break down before the time of pollination. The pollen mother- 

 cells are in synapsis at the time when the ovules first appear (figs. 

 1-2). The somatic number of chromosomes, twelve, is reduced to 

 six probably in the first mitosis of the pollen mother-cell ; unfor- 

 tunately the metaphase of this mitosis was not available. Six chro- 

 mosomes apj)ear at each pole of the heterotypic diaster (fig. 4) 

 and in the homoty]:)ic metai^hase (figs. 5-8). Each mitosis is fol- 

 lowed immediately by the development of a cell wall so th^it the 

 mother-cell is first divided into two hemispherical daughter cells 

 and then into four bilateral grand-daughter cells. These two sec- 

 ond dividing walls may meet each other at any angle. The pollen 

 grains soon become separated. The nucleus (fig. 10) divides be- 

 fore pollination (fig. 11), and the grain becomes shaped like a 

 shortened wheat kernel. 



The night-blooming flowers open at sunset, each flower open- 

 ing on several successive evenings. They are pollinated within 

 a few hours after the flower opens. Their pollination by the moth 

 Pronuba yuccasella has been carefully observed by Riley.'' The 

 mass of pollen is deposited in the end of a tube in the center of the 

 stigma and there the pollen grains germinate before sunrise. 

 Within two to four days after pollination, the pollen tubes grow 

 down through the open stigmatic tube, through its three branches 

 which lead to the three cavities of the ovary, along the placentae and 

 the funiculi of the ovules, and through the micropyles. They pass 

 through a granular substance filling the micropyle and covering 

 the adjacent part of the fvmiculus (fig. 31). This substance is seen 

 from just before anthesis until the pollen tube reaches the embryo 

 sac, and may contain chemotactic substances which direct the course 

 of the tube ; a similar condition in Yucca filamcutosa L. is reported 

 by Reed.^ The tube usually decreases in diameter as it pierces 

 the layer of nucellar cells, and discharges its contents into the em- 

 bryo sac without going beyond this layer (figs. 33. 34). 



Fertilization 



Two ellipsoidal male nuclei are discharged from the pollen tube 



