108 OUTLINES OF EVOLUTIONARY BIOLOGY 



It- 



of germination of this spore in typical cases (Fig. 54) is 

 marked by the division of the nucleus into two. Around one of 

 these two cytoplasm collects to form a naked " antheridial cell " 

 (HI) ; the other, with the remainder of the cytoplasm, constitutes 

 the " vegetative cell " (k), which may or may not divide again. 

 The antheridial cell divides into two " generative cells." The 

 vegetative cell, or cells, represents the last vestige of the 

 body of the male prothallus ; the generative cells are male gametes. 

 The germination of the pollen grain and development of the 



male prothallus are completed by 

 the putting forth of the pollen tube 

 (Fig. 53, ps, Fig. 54), which takes 

 place if the pollen grain is fortunate 

 enough to alight upon the stigma of 

 a flower of the right kind. The 

 pollen tube forces its way through 

 the loose tissue of the style to the 

 ovary and comes into intimate rela- 

 tions with one of the ovules contained 

 therein. * 



The female gametophyte is repre- 

 sented by a few cells formed by 

 division of the rnegaspore (embryo 

 sac), or rather of its nucleus, and to 

 some extent of its cytoplasm, within 

 the ovule. The process is a some- 

 what complicated one, but, without 

 going into details, we may note that 

 at the time when the ovule is ready 

 for " fertilization " the embryo sac in a 

 typical flowering plant (Angiosperm) contains seven cells, one of 

 which is a female gamete (ovum or oosphere), while the others 

 may be taken to represent the female prothallus, including a 

 vestige of an archegonium. The arrangement of these cells is 

 shown in Fig. 55 (at, <>, k, s). 



The embryo sac or megaspore (E) is surrounded by a cellular 

 layer known as the nucellus (Fig. 55, K), which represents the wall 

 of the sporangium, and this in turn by two other coats (ai and ii), 

 the outer and inner integuments of the ovule, which grow up 

 around the nucel-lus. The entire ovule is attached to the wall 

 of the ovary by a stalk or funiculus (/), upon which it is 



FIG. 54. Germination of the 

 Pollen Grain of Lilium 

 martagon, X 375. (From 

 Strasburger, after 

 Guignard.) 



k, nucleus of the vegetative cell of 

 the pollen tube ; m, antheridial 

 cell ; g, male gametes formed 

 by division of the antheridial 

 cell. 



