SPERMATOPHYTES—ANGIOSPERMS 
303 
orchids, in which a common exine surrounds the entire mass of pollen 
grains, care must be taken not to overstain. 
In many cases the pollen grains will put out their tubes in a 2 to 
5 per cent solution of cane-sugar in water. Where the interval 
between pollination and fertilization is known (about 72 hours in 
Lilium philadelphicum and 96 to 100 hours in L. canadense), pieces 
of the stigma and style showing pollen tubes can be selected with 
some certainty. 
Oogenesis.—As in spermatogenesis, the early stages will be found 
in preparations of floral development. The preparations of Capsella 
will show the origin and development of the nucellus (megaspo¬ 
rangium) and also the megaspore mother-cell. The division of the 
megaspore mother-cell to form four megaspores takes place shortly 
before the bud begins to unfold. A massive megasporangium with 
several megaspore mother-cells may be found in Ranunculus; a 
megasporangium with only one megaspore mother-cell and only one 
layer of cells surrounding it may be found in any of the Compositae. 
Senecio aureus and Erectites hieracifolium are good and are particu¬ 
larly easy to cut. In Trillium and in Cypripedium the embryo sac is 
formed from two megaspores, which are not separated by walls. In 
Lilium , Tulipa, Fritillaria, Erythronium, and many others, the embryo 
sac is formed by alb four megaspores, which are not separated by 
walls. In Peperomia, the Peneaceae, and some species of Euphorbia, 
the sac is formed by the four megaspores, not separated by walls, and 
the sac has 16 free nuclei. In Plumbagella the four megaspores, 
not separated by walls, constitute the mature sac, one of the mega¬ 
spores functioning as the egg, two more fusing to form the endosperm 
nucleus, while the fourth megaspore aborts; so that the embryo sac, 
ready for fertilization, contains only two nuclei. 
The reduction of chromosomes takes place during the two mitoses 
by which the mother-cell gives rise to four megaspores. The figures 
are much larger than in the corresponding mitoses in spermatogenesis 
but so much more tedious to secure that most studies in reduction 
have been based upon divisions in the pollen mother-cell. Lilium is 
quite favorable for a study of oogenesis, but it must be remembered 
that it is exceptional in having an embryo sac formed from four 
megaspores. 
In very young stages, before the appearance of the integument, 
the ovary may be removed from the flower and placed directly in the 
