244 



GENERAL BIOLOGY 



portion moving toward opposite poles of the megaspore cavity. Each 

 of these portions divides twice, thus forming four nuclei at each pole. 



One nucleus from each pole (often called the polar nuclei) then 

 moves toward the center and these two meeting, unite. 



One of the nuclei about the pole functions as an egg nucleus. The 

 two companion cells are called synergids. The cells at the opposite pole 

 are called the antipodal cells. 



It is at this time that the pollen, which has fallen on the carpel 

 stigma, germinates to produce a reduced gametophyte and a pollen tube. 

 This pollen tube penetrates the soft stigma tissues and carries two sperm 

 toward the ovary cavity. As the pollen tube reaches the ovule it enters 

 a tiny pore called the micropyle between the two integuments, and then 

 passes through the nucellus. The ovule is thus penetrated, and one of 

 the sperm unites with the egg and fertilizes it. 



The zygote now divides continually and soon there is developed a 



A. 



Fig. 148. 



A. At the left, diagram of the anatomy of an angiospermous flower shortly 

 after pollination ; anth., anther ; fil., filament ; st., stamen ; stig., stigma ; p. g., 

 pollen grains germinating; sty., style; pt., pollen tube; o. w., ovary wall; o., ovule, 

 containing embryo-sac; pet., petal; sep., sepal. 1-8, Stages in the development of 

 the female gametophyte (embryo-sac) ; meg.sp., megaspore-mother-cell ; i.L, inner 

 integument ; o.i., outer integument ; fun., funiculus ; chal., chalaza ; nu., nucellus 

 (megasporangium) ; emb., embryo-sac. All diagrammatic. (From C. Stuart 

 Gager's "Fundamentals of Botany," by permission of P. Blakiston's Son & Co. 

 Publishers). 



B. Diagrammatic Representation of Fertilization of an Ovule. 



t, inner coating of megasporangium (ovule) ; o, outer coating of ovule; p, 

 pollen tube proceeding from one of the pollen grains on the stigma ; c, the place 

 where the two coats of the ovule blend. (The kind of ovule here shown is inverted, 

 its opening m being at the bottom, and the stalk / adhering along one side of the 

 ovule), a to e, embryo sac, full of protoplasm; a, so-called antipodal cells of 

 embryo sac ; n, central nucleus of the embryo sac ; e, nucleated cells, one of which, 

 the egg cell, receives the male nucleus of the pollen tube ; /, funiculus or stalk of 

 ovule; m, micropyle or opening into the ovule. (After Luerssen). 



