428 MINNESOTA BOTANICAL STUDIES 



actively growing tissues and usually produced excessive shrinking 

 in the embryo sac. The usual paraffin method, with xylol as a clear- 

 ing agent, was used in making the preparations. The sections, 

 which were from 4 mic. to 10 mic. in thickness, were stained best by 

 Haidenhain's iron-alum haematoxylin and by the triple stain — anilin- 

 water safranin, gentian violet, and orange G — used according to 

 Harper's short method. Drawings were made with the aid of an 

 Abbe camera lucida using a Bausch and Lomb microscope, a com- 

 bination of a 1.9 mm. "semi-apochromatic" oil-immersion lens and 

 a No. 12 compensating Zeiss ocular, and other combinations giving 

 less magnification. 



General Development of Ovule and Embryo Sac 



The youngest ovaries examined contain in each an axial six- 

 rayed cavity which becomes closed by the almost complete grov.ang 

 together of the three carpels (Plate XLIII, fig. 13). Thus three 

 loculi are formed whose upper ends unite just above the ovuliferous 

 portion to form an open tube ending in the center of the stigma. 

 In each loculus two rows of ovules begin to protrude from the 

 placentae (fig. 13). 



While the ovule is becoming inverted, the inner integument 

 pushes out, followed by the outer (fig. 16). These elongate — the 

 outer one also increasing considerably in thickness — until at anthesis 

 they completely enclose the nucellus except for the rather open 

 micropyle (fig. 19; fig. 24, ovule .45 mm. long; fig. 31; fig. 35, 

 ovule .93 mm. long). After fertilization the growth of the integu- 

 ments continues, accompanying the growth of the embryo sac (fig. 

 38, ovule 4 mm. long ; fig. 40, ovule 8 mm. long) , the micropyle be- 

 coming closed entirely in six to eight days. 



When the integuments have become nearly as long as the nucel- 

 lus, the latter contains a cylindrical to ovoid group of megaspores 

 (figs. 20-21, group 63 mic. x 35 mic. ; fig. 22). The embryo sac soon 

 comes to fill the whole space formerly occupied by this group of 

 megaspores. It grows uniformly in length during the entire develop- 

 ment of the ovule until it reaches its mature size in the nearly ripe 

 seed. For a time it maintains a nearly proportional growth in all 

 dimensions, but some time before anthesis its growth in width grad- 

 ually ceases at the chalazal end while continuing in the more distal 

 portion. As a result the embryo sac becomes first more or less 



