Flowers. 



167 



shaded with diagonal lines) 

 either through thin places de- 

 signed for the purpose, or 

 through crevices caused by 

 swelling of the inner parts, and 

 grows forth in the form of a 

 slender tube (/). This pene- 

 trates between the cells of the 

 tissues of the style, from which 

 it derives nourishment, until 

 it reaches the cavity («) of 

 the ovary, in which the ovule 

 (y) is contained, and then it 

 seeks the opening (micropyle) 

 through the coats of the ovule, 

 and, passing through this, 

 reaches the spore (s) called 

 the embryo sac, in which the 

 egg {k) is borne. Then the 

 pollen tube breaks open, and 

 the sperm («), which has been 

 passing down within the tube 

 as it elongated, fuses with the 

 egg (see Fig. 89, where h is 

 the egg cell, t, pollen tube, and 

 u the uniting sperm and egg 

 nuclei). This fusion of the two 

 sexual elements is called the 

 fertilization of the egg. 



The egg and the sperm are 

 both protoplasts, — that is, they 

 both consist of plasma mem- 

 brane, cytoplasm, nucleus, and 



Fig. 88. 



Showing the descent of the Pollen 

 tube and its entrance into the Ovule. 

 OT, pollen grain on the stigma ; /, the 

 pollen tube ; n, the sperm cell which 

 has descended the tube and is ready 

 to penetrate the embryo sac, j, and 

 fuse with the egg cell, k. Below k 

 are two cells termed the synergids ; 

 e, two nuclei which fuse and form 

 the secondary nucleus of the em- 

 bryo sac, from the division of which 

 the tissue of the endosperm arises ; 

 g, the nucellus; f, antipodal cells; 

 2. outer, and y, inner coat of the 

 ovule ; u, cavity of the ovary. 



