No. 501] EMBRYOLOGY OF MYOSURUS MINIMUS 583 



or twice, producing three or four layers in the wall. The 

 latter cuts off spore mother cells which take the place of 

 the degenerating meristem. The tapetal layer appears 

 to be the outer layer of spore mother cells which have 

 sacrificed their reproductive function for the nourishment 

 of the other cells (PI. I, Pig. 2). In this earliest stage 

 of tapetal differentiation, some of the tapetal cells are 

 binucleate. A little later practically all of them become 

 binucleate and remain so till degeneration is accomplished 

 (PI. I, Fig. 3). This fact, to my mind, supports the 

 theory that tapetal cells are potentially spore mother 

 cells. The hereditary stimulus toward further division 

 to form microspores is strong enough in them to carry 

 the nucleus through one division, at which point their 

 new function usurps control and arrests further develop- 

 ment. 



The mother spores are few in number and as the 

 anther enlarges become rounded off and separated from 

 each other (PI. I, Fig. 3). The usual course of develop- 

 ment now follows. The mother cells divide simulta- 

 neously in a given sporangium to form first two and then 

 four cells arranged either tetrahedrally or in the same 

 plane (PL I, Fig. 4). The last three stages in the 

 maturity of the microspores are shown in Figs. 5, 6 and 7. 

 There are developed on the wall three thin spots for the 

 extrusion of the tube. The tube nucleus, the round one, 

 and the generative nucleus, the oval one, are always 

 present before the pollen leaves the anther. Upon germi- 

 nation the generative nucleus evidently divides, for two 

 male nuclei are found in the pollen tubes. Degeneration 

 of the tapetum is not very marked till the pollen grains 

 are completely formed. In the mature anther not only 

 the tapetum but all the other cells of the wall except the 

 epidermis and first hypodermal layer have disappeared. 

 This hypodermal layer develops heavy bands in the cell 

 walls for breaking the anther. 



The pistil appears in its earliest stage as a small 

 swelling on the side of the receptacle (PL I, Figs. 8-9). 



