FEETILIZATION. 



143 



these new cells cohere into a tissue assuming a definite form, as 

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

 separate, each one still abiding separate, it is reproduction. 



449. The embryonic vesicle is the expressive name of the 

 embryonic cell of the Flowering 

 Plants. It has its birth in that 

 large cell of the nucleus of the 

 ovule ( 142) called the embryo 

 sac, and is in some way developed 

 from the cytoblast ( 380). In 

 appearance it may be like other 

 new cells ; but in the impulse or 

 instinct with which it is endowed, 

 it is immeasurably different. It 

 looks not to the mere continuation 

 of an old series, but is the projector 

 and pioneer of a new. But before 

 it can enter upon its course of de- 

 velopment so different from the 

 destination of common cells, it 

 must somehow be quickened and 

 energized with an impulse in this 

 new direction. In other words, 

 it must be fertilized a process 

 dependent on the pollen-grains 

 (,121). 



450. The annexed cut indicates all that is cer- 

 tainly known of this recondite process. The 

 pollen-grain falls upon the stigma, imbibes the 

 saccharine moisture there, and its inner coat 

 of protoplasm expands and protrudes through 

 the aperture (one or more) of the outer crusta- 

 ceous 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, enters the micropyle of the ovule, makes its 

 way to the nucleus, and penetrates to the embryo sac. Here its growth ceases, and ita 

 contents pass by absorption into the sac. 



451. This is the view of Mohl, Henfrey, and of botanists generally. But Schleiden 

 maintains that the end of the pollen-tube actually penetrates the sac, and itself becomes 

 the embryonic cell. The pollen-grain is in this view the primitive cell, and is itself quick- 

 ened into development by the contents of the embryo sac (522). 



452. However this may be, the embryonic globule, thus some- 



521, Section of the ovary of Polygonnm Perm 

 sylvaiiicum, in process of fertilization. (Mag- 

 nified 20 diameters.) r, Natural size; , one of 

 the stamens, having discharged its pollen; t, a 

 grain of pollen and its tube; .1, styles and stig- 

 mas o, ovary, ovule, embryo sac containing the 

 embryonic globule. The extremity of a pollen- 

 tube is seen in contact with the embryo sac. 



