SPERM vmi'iivi i:s 



'75 



618). Notable variations in this 

 method of embryo formation arc 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- 

 ment of an embryo from an unfer- 



Fig. 618. — Stem tip of Sagittaria de- 

 veloping in notch on 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 Thalictrnm, 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- 



