FERTILIZATION. 



149 



the higher plants, the process is called growth ; but if they separate, 

 each one still abiding separate, it is reproduction. 



754. THE EMBRYONIC VESICLE is the expressive name of the em- 

 bryonic cell of the Flowering Plants. It has its birth in that large cell 

 of the nucleus of the ovule ( 539) called the embryo sac, and is in 

 some way developed from the cytoblast ( 655). In appearance it may 

 be like other new cells, but in the impulse <>r instinct with which it is 

 endowed it is immeasurably different. It looks not to the mere con- 

 tinuation of an old series, but is the projector and pioneer of a new. 



755. ITS NEW IMPULSE. Before it 

 can enter upon its course of develop- 

 ment so different from the destination 

 of common cells, it must somehow be 

 quickened and energized with an im- 

 pulse in this new direction. In other 

 words, it must be fertilized, a pro- 

 cess dependent on the pollen grains 

 ( 509). 



756. THE POLLEN TUBE ITS COURSE. 

 When the pollen falls upon the stigma, it 

 imbibes the saccharine moisture there, ex- 

 pands, and its inner, expansible coat of pro- 

 toplasm protrudes through the aperture (one 

 or more) of the outer crustaceous coat, in 

 the form of an attenuated tube. This, like 

 a radicle, sinks into the soft tissues of the 

 stigma and style, reaches the ovary, and 

 there meets and enters the micropyle of the 

 ovule. 



757. ITS CONTENTS, HOW DISCHARGED. 



At this juncture the ovule has so turned it- 

 self, whether orthotropous, anatropous, etc., 

 as to present the micropyle favorable to this 

 process. The pollen tube makes its way 

 finally to the nucleus and penetrates to the 

 embryo sac. Here its growth ceases; its 

 point is applied externally to the sac, some- 

 times indents it ; but (according to the most 

 accurate observations), does not penetrate 

 it During this contact the contents of the 

 tube pass by absorption into the sac. 



758. GROWTH OP THE FERTILIZED CELLS. 

 Immediately the embryonic globule, thus, 

 somehow endowed with a new instinct, now 



607, Section of the ovary of Polygonum 

 Pennsylvanicurn, in process of fertilization. 

 (Magnified 20 diameters), c, Natural size. 

 ., One of the stamens having discharged its 

 pollen, t, A grain of pollen and its tube. 

 , Styles and stigmas, o, Ovary, ovule, em- 

 bryo sac containing the embryonic globule. 

 The extremity of a pollen tube is seen in 

 contact with the embryo sac. 



