SPERMATOPHYTES 
275 
618). Notable variations in this 
method of embryo formation are as 
follows : 
Variations. Among the aroids 
the proembryo is usually a spherical 
mass of cells. 
In Lilium and its allies the sus- 
pensor becomes massive, sometimes 
filling the micropylar end of the sac, 
and occasionally giving rise to extra 
embryos. 
In orchids the embryo is very 
simply organized at the maturity of 
the seed, consisting of only a few 
cells, with no differentiation into 
body regions. 
PARTHENOGENESIS 
Parthenogenesis is the develop- 
- , , FIG. 618. Stem tip of Saeittaria de- 
ment of an embryo from an unfer- vdoping in notch oa side of embryo- _ 
tilized egg, and it seems to be a rare After SCHAFFNER. 
phenomenon among angiosperms, 
having been recorded thus far in only six or eight scattered genera, 
conspicuous among which are species of Thalictrum, Alchemilla, Anten- 
naria, and Taraxacum. In these cases, since there is no fusion of 
nuclei in the act of fertilization, there is no doubling of the chromo- 
somes; but it has also been found that in them there is no reduction 
division in the formation of the megaspores, so that the egg already 
has the 2X number, which is transmitted to the young sporophyte 
(embryo). 
POLYEMBRYONY 
The occurrence of more than one embryo in a seed is not so common 
among angiosperms as among gymnosperms, but the cases are numer- 
ous. The synergids and the antipodals have been observed to produce 
embryos, and since these cells are gametophytic, these embryos arise 
by vegetative apogamy (see p. 169). Very commonly also the cells of 
the nucellus adjacent to the embryo sac or even those of the integument 
may form embryos that push into the sac. Since the nucellus and in- 
