Dec., 1907.] 
Life History of Asimina triloba. 
213 
now divide rapidly 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 3 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 sections 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. i: wiss. Botanik. 37 : 477-526. Pis. 
■9-11. 1902. 
