423 



BOTANY. 



532. Fertilization. — Fertilization takes place as follows : 

 The pollen grain, resting upon the moist surface of the 

 stigma, absorbs moisture and germinates, sending out a tube 

 which penetrates the soft tissues of the stigma and style, 

 finally reaching the cavity of the ovary, where it enters the 

 micropyle of an ovule (Fig. 307, A). Here it comes in con- 

 tact with the apex of the ovule body, through whose tissues 

 it forces its way until it reaches the embryo sac ; in some 



cases, however, the 

 embryo sac has grown 

 out through the apex 

 of the ovule body 

 into, and occasionally 

 through the micro- 

 pyle, thus meeting the 

 pollen - tube. The 

 transfer of the con- 

 tents of the pollen- 

 tube to the germ-cell 

 has never been ob- 

 served, but doubtless 

 it takes place by diffu- 

 sion through the pol- 

 len-tube and embryo 

 sac. The first result 

 of fertilization is the 

 formation of a wall of 

 cellulose around the 

 the latter 

 trans- 



Fig. 307. — A, a longitudinal section of the anatro- 

 poue ovule of Viola tricolor^ after fertilization, pl^ 

 the placenra ; w. the raphe, swollen at this point ; a, 

 the outer coat of the ovule ; i, the inner ; », the poi- 

 len-tube which has entered the micropyle ; e, em- 

 bryo sac, with the very young embryo at the micro- 

 pylar end, and numerous free endosperm cells at the 

 other, .ff, apex of embryo sac, e (mnch more mag- 

 nhied) ; e6, very young embryo of two cells, support- 

 ed by a two-celled suspensor. C, the same, further 

 advanced. All the figures highly magnified.— After gCrm-cell 

 Sachs. , . . , 



soon divides 



versely one or more times, and thus gives rise to a row of 

 cells, the suspensor, at the free extremity of which a rudi- 

 mentary embryo is soon formed by the fission of cells in 

 three planes (Fig. 307). Simultaneously with the foregoing 



ized by Insects ; " " The Eflfecta of Cross and Self-Fertilization in the 

 Vegetable KingdonT;" "Tlie Different Forma of Flowers on Plants of 

 the Same Species." Also Lubbock's " British Wild Flowers Considered 

 in Relation to Insects," and Dr. Gray's " How Plants Behave." 



