Dec, 1907.] Life History of Asimina triloba. 213 



now divide rapidty as indicated by Figs. 13 and 16, forming a 

 linear series of endosperm of about a dozen cells, with transverse 

 walls, when a vertical wall appears in the base of the sac (Fig. 16). 

 The endosperm grows rapidly until there is formed a narrow 

 strip throughout the length of the seed, the upper end of which 

 is shown in Fig. 17. The order of the endosperm division was 

 not determined but the indications are that the divisions are not 

 basipetal, as Strasburger^ found for Ceratophyllum, though the 

 first division of the sac into two halves is similar. Figs. 12, 13 

 and 16 show what has taken place. Apparently the endosperm 

 divides into two cells, each of which divides again and so on until 

 perhaps a dozen -cells have been formed in a linear series when 

 vertical division takes place as already noted. 



The germination of the oospore is very much delayed, as 

 has already been pointed out. The first wall is transverse and 

 the subsequent divisions quite irregular. The embryo in the 

 mature seed is very minute and imperfectly developed, the 

 greater part of the seed being occupied by a peculiar wrinkled 

 tissue of the wall of the ovule. Fig. 17 (June 10) shows the upper 

 end of the endosperm column with the minute embryo and rem- 

 nants of the pollen tube. 



MiCROSPORANGiUM AND MICROSPORE. The first scctious made 

 (Sept. 30) showed the differentiation of the microsporophylls 

 each with four microsporangia. Fig. 18 shows one of the mic- 

 rosporangia in which may be seen a number of microsporocytes 

 in cross section, of which two show a somewhat greater develop- 

 ment and more prominent nuclei. This condition is not altered 

 throughout the winter as is shown by Fig. 19 of Jan. 6. Fig. 20, 

 of March 10, shows the growth of the microspores at that date, 

 being the earliest to show what is really taking place. It can 

 be seen at once that some of the sporogenous tissue is breaking 

 down and that only a few; microsporocytes (usually two in cross 

 section) are building up and growing at the expense of the sur- 

 rounding cells. Fig. 21 (April 14) is interesting since here may 

 be seen the differentiation of a bridge of tissue between the 

 microsporocytes. The nuclei of these cells have divided as well 

 as those of the surrounding tapetal cells. By April 21 (Fig. 22) 

 the spore tetrads are formed and the sterile tissue in the sporan- 

 gium is dissolving rapidly. By April 28 (Fig. 23) the sterile 

 tissue and tapetum has completely dissolved. May 5 (Fig. 24) 

 the pollen grains are ready to be shed, the generative and tube 

 nuclei being formed. 



3. Strasburger, Eduard, '02. — Ein Beitrag zurKenntniss von Ceratophyllum submer- 

 sum und phylogenetische Eroerterungen. Jahrb. f. wiss. Botanik. 37 : 477-526. Pis. 

 S-11. 1902. 



