Feb., 1903.] Agrostemma githago. 365 



THE DEVELOPMENT OF THE EMBRYO-SAC AND 

 EMBRYO OF AGROSTEMMA GITHAGO.^^ 



MELviivi<E Thurston Cook. 



Agrostemma githago L. was selected as a plant for comparison 

 with Claytonia virginica, they usually being placed in tlie same 

 order but in different families. 



The ease with which the material can be prepared and the reg- 

 ularity of the development of the embr3'o-sac and embryo make 

 it a very desirable plant for study. 



METHODS. 



The material was killed and fixed in Flemming's solution and in 

 chromo-acetic, passed through the alcohols, imbedded in paraffin, 

 cut on a Minot microtome and stained in aniline safranin and 

 gentian violet, and also in Hadenhaine's haematoxylin and iron 

 alum. For the development of the embryo-sac the safranin and 

 gentian violet combination was most desirable, but for the devel- 

 opment of the embryo either stain was satisfactory. 



The young buds were killed entire, but the sepals and petals 

 were removed from the older ones. In the ver}- youngest stages 

 it is desirable to cut the sections thin, but when the embrj-o-sac 

 has reached the eight-celled stage it must be cut thick, otherwise 

 important structures may be lost. The orientation was very 

 simple ; by cutting the ovaries transverely the sac will be cut 

 longitudinally. 



The archesporium may originate as one, but more frequenth- 

 as two or three, hj-podermal cells (Figs, i, 2, 3). These increase 

 in size (Fig. 2), and one eventually absorbs the others. Man}- 

 specimens were examined, but in all cases only one cell developed 

 into an embryo-sac. This single archesporial cell now divides by 

 transverse divisions into three cells, of which the lowest develops 

 into the functional megaspore (Fig. 4.) The two (Fig. 5), four 

 ( Fig. 6), and eight (Fig. 7) celled stages of the embryo-sac are 

 formed in the usual manner. The sac increases in size very 

 slowly up to this time, and the nuclei of the sac are of practically 

 the same size (Figs. 6 and 7), except that antipodals are slightly 

 smaller than the other nuclei. 



After the formation of the megaspore the ovule begins to 

 enlarge, and a very pronounced growth of the nucellus and integu- 

 ments on the micropolar side projects from the micropyle. The 

 embryo-sac is thus left deeply imbedded in the nucellus (Figs. 4 

 and 23). By the time the sac has reached the two-celled stage 

 the nucellus shows two well-defined zones (Figs. 5 and 23). The 

 inner zone surrounding the sac is made up of thin-walled cells, 



* Coutributioii from the Botanical I^aboratory of Ohio State University. Xll 



