Nov., 1907.] Life History of Cornus Florida. 



20I 



pollen. The grains when shed are very much Hke the two-celled 

 pollen grain of April 30th (Fig. 13), except they are more elon- 

 gated, being sub-spindle shaped. In 1907, although the season 

 was somewhat late, the shedding period was about over on 

 May 27th. Fertilization was not observed. 



Those flowers which were fertilized, however, could easily be 

 distinguished external!}^ by their increase in size over the others. 

 Of the fertilized flow^ers there were usually from one to three 

 upon a head. Those receptacles which contained no fertihzed 

 flowers were soon shed by the formation of cleavage-planes in 

 the peduncles. Great numbers of the heads thus cut off could be 

 gathered under a single tree. 



ENDOSPERM AND EMBRYO. 



The first material after the last eight-celled embryo-sac 

 (May 21st) was collected June 12th. By this time there had 

 been a rapid development of the endosperm. This filled the 

 upper third of the embryo-sac, extending up to where the S3nier- 

 gids lay. The contorted, double pointed cone of the synergids 

 at this time still retains its characteristic shape. The tissue of 

 the integument is beginning to break down. 



On June 20th the endosperm had pushed its way in the upper 

 end of the sac practically to the apex of the cone. In the lower 

 half were a large number of loose endosperm cells. By June 

 29th, the endosperm had reached a very characteristic arrange- 

 ment. The upper part forms a cap-like structure, the cells of 

 which are arranged concentrically. The integument is nearly 

 destroyed, there being merely strands of disintegrating cell walls. 



In the material of June 12th, 20th and 29th, appeared a short 

 chain of' irregular cells whose walls were much thicker than those 

 of the cells of the surrounding endosperm. Whether or not these 

 cells were the young embryo was not determined. The first 

 material which, without question, contained a 3^oung embryo was 

 taken Juh^ 9th. At this time the embryo is a spherical mass of 

 cells suspended by a suspensor from the cap of endosperm cells 

 (Fig. 21). The endosperm cells do not as yet completely fill the 

 embryo-sac and fragments of the integument cell walls remain. 



The embryo has developed rapidly by July 21st. On this 

 date the two cotyledons are present (Fig. 22). Some of the 

 cells are becoming differentiated to form the vascular cylinder. 

 The endosperm completely fills the sac except for the small 

 cavity in which the embryo lies. 



The last section was made from material collected July 2Sth. 

 In this the embryo had become quite large (Fig. 23)'. The 

 cotyledon, h3^pocotyl, root tip, root cap, and embryonic tissues 

 were well differentiated. The endosperm,- as on July 21st. 

 completel}^ filled the sac. 



