1919I EVANS— PENTSTEMON 435 



when one considers that it covers only the chalazal end of the sac in 

 a number of species. Surely the micropylar end would be as much 

 in need of protection. In P. secundiflonis the integumentary cells 

 border on the micropylar end of the sac. 



The two haustoria function in passing nourishment to the endo- 

 sperm cells which are farther from the supply of food. By the 

 time the embryo has reached the endosperm the micropylar haus- 

 torium becomes inactive and is lost. The chalazal one, however, 

 functions until the endosperm is formed. The nuclei in this 

 haustorium are very pronounced. On account of the large size 

 and seeming activity of haustorial nuclei some authors have 

 attributed to them si considerable role in nutrition. Balicka- 

 IwANOWSKA (2) has always found them near the point where 

 nutrition is most abundant. In this work a similar tendency was 

 noted. 



The growth of the suspensor in such a manner as to push the 

 proembryo through the micropylar end of the embryo sac and to 

 imbed it in the endosperm is rather unique. Sharp (13) in his 

 study on Physostegia has recorded a similar situation, but the 

 method of endosperm formation in Physostegia and Pentstemon 

 is different entirely. 



During the growth of the suspensor which imbeds the embryo 

 in the endosperm, nutrition is derived from the starch stored up in 

 the micropylar end of the sac. 



Summary 



1. The embryo sac is developed from a single megaspore. Its 

 antipodals disorganize early. The micropylar end becomes bul- 

 bous, while the chalazal end becomes long and narrow and is 

 covered by a distinct tapetum. 



2. The mature embryo sac is found to be constantly gorged 

 with starch, due to the non-utilization of the nutritive materials 

 which pass into the sac at a time of inactivity just before^ 

 fertilization. 



3. The endosperm nucleus immediately divides and free nuclei 

 migrate into the chalazal end of the sac, where wall formation 

 begins. The proembryo is pushed into this endosperm by an 



