Nov., 1907.] Life History of Cornus Florida. 199 



in turn divided until there are four parietals, and at least tAvo 

 radial sporogenous layers. 



Material collected August 26th showed that in the youngest 

 flowers the three outer parietal layers remain thin and flat while 

 the inner has enlarged and functions as the tapetuni. The 

 sporogenous tissue has reached the microsporocyte stage, the 

 nuclei being in synizesis (Fig. 8). Very commonly the chromatin 

 is in a contracted mass on one side of the nuclear cavity while the 

 nucleolus lies free on the other side. 



In the oldest flowers of this date the tapetal layer is much 

 broken up into individual cells which are binucleate. The 

 microsporocytes have divided twice in rapid succession without 

 forming a cell wall between them. The result is the spore 

 tetrad within the microsporocyte wall (Fig. 9). 



On September 2d the cells of the tetrad had separated into 

 the individual spores. The microspores are somewhat elongated 

 with three double ridges upon their surface. 



Commencing September the Sth a gelatinous mass is forming 

 within the microsporangia. On September 24th it had grown 

 much more dense and on the 26th of the same month of the 

 previous year the same condition was present. As this matrix 

 (Fig. 12) remains within the sporangia throughout the winter it 

 no doubt functions as a protection to the microspores and pollen 

 grains. In the spring it begins to dissolve in some sporangia the 

 last of February while in others it remains until the last of April. 



At just what date the microspores divide to form the two- 

 celled pollen grain (male gametophyte) was difficult to determine 

 because of the deep stain which the gelatinous matrix takes. It 

 was found, however, that the two-celled stage was present 

 December 4th. In this stage, then, they pass the winter, the 

 male gametophyte being developed the fall previous to the 

 blooming of the flower. 



THE DEVELOPMENT OF THE FEMALE GAMETOPHYTE. 



As has been stated, the incipient ovule makes its appearance 

 about August 11th as a papilla on the wall of the stylar canal. 

 It grows downward and then bends upon itself becoming anatro- 

 pus August 18th. On this same date the single integument 

 makes its appearance. On August 18th, less than one month 

 after the appearance of the first floral organs, the sepals, the 

 hypodermal archesporial cell is much elongated and contains a 

 large nucleus. This cell becomes the megasporocyte (Fig. 14). 



One week later, August 26th, two subsequent cell divisions 

 have occurred. The result is the four megaspores (Fig. 15). Of 

 these the three apical ones are small and non-functional, while 

 the other, functional one is five or six times as large. No cell 

 Avails were seen between them. 



