Folsoni: morphology of yucca glauca 429 



wedge-shaped (figs. 27, 29), and later, a short time before anthesis, 

 club-shaped with a narrow antipodal portion (fig. 30, 55 mic. x 30 

 mic.) and an enlarged micropylar portion (fig. 30, 145 mic. x 110 

 mic.) Disintegration of part of the nucellar tissue around the 

 micropylar end of the embryo sac begins with the first stages in the 

 formation of the embryo sac and is complete at the time of flower- 

 ing (figs. 25-30). 



Ovule Previous to Fertilization 



When the rudiment of the ovule protrudes from the placenta, an 

 archesporial cell is differentiated (fig. 14), being larger than the 

 other cells, just below^ the epidermis at the distal end of the nucellus. 

 The archesporial cell divides periclinally into a small tapetal cell and 

 a large megaspore mother-cell (fig. 15). The former later divides 

 once or twice anticlinally. The latter increases in size (fig. 16) and 

 goes through two successive divisions, giving rise to four mega- 

 spores by the time the integuments have become as long as the 

 nucellus (figs. 17-21). The somatic number of chromosomes, 

 twelve, is reduced to six in the first of these mitoses (figs. 17-18). 



There are three general types of arrangement of the megaspores. 

 All four may lie in a row in the axis of the nucellus or either the 

 two micropylar megaspores or the two middle ones may be side by 

 side. In the latter cases the two which lie side by side appear to 

 have no definite orientation in respect to the axis of the ovary, as in 

 cross-sections of the ovary they sometimes appear in the same plane, 

 and sometimes with one directly or obliquely behind the other. The 

 most common type of arrangement is the second. The relative fre- 

 quency of occurrence of the three types may be seen in table I. 



Three of the megaspores break down (figs. 22, 23), while the 

 fourth develops into the embryo sac. The functional spore may be 

 any one of the group, and about an equal number of instances were 

 observed in which the distal, a proximal, or one of the central spores,, 

 respectively, were developing (see table I). The functional mega- 

 spore (figs. 23-25) germinates in the usual way, forming eight 

 nuclei by three successive divisions, of which the second and third 

 occur near the poles of the embryo sac (figs. 26-28). The fusion 

 of the two polar nuclei takes place near the antipodal end of the 

 embryo sac before the flowers open (fig. 30). 



