SPERMATOPHYTES—ANGIOSPERMS 
311 
Sagittaria has been used quite extensively. It is easily obtained, 
the whole head can be cut with ease, even after the cotyledon and 
stem-tip are clearly differentiated, and the endosperm is instructive; 
but the embryo is curved, like that of Capsella, and good views are 
rather rare. 
Zea Mays , especially the sweet corn, is a good type to illustrate 
the peculiar embryo of the grasses. Directions have been given on 
page 294. 
In many forms good preparations of late stages may be secured 
by soaking the seeds in water until the embryo bursts the seed coat. 
Young seedlings furnish valuable material for a study of vascular 
anatomy. 
Parthenogenesis. —Many embryos are developed without fertili¬ 
zation. Taraxacum , the common dandelion, is an omnipresent 
example. Other widely distributed illustrations are Hieracium 
and practically all species of the Eualchemilla section of the genus 
Alchemilla. Parthenogenetic forms show various irregularities in the 
mitoses leading up to the formation of the egg. 
