84 STUDY OF COMMON PLANTS. 



recall the seed, which is simply a fertilized and matured 

 ovule. 



When pollen-grains have been brought by any agency 



to the moist and receptive stigma of a flower of the same 



species, they begin after a short interval to ger- 



Fertilization, . T . n ^ i 



minate. In germination pollen-tubes are pro- 

 duced, which rapidly elongate, growing through the loose 

 tissue of the stigma and downwards through the style until 

 they enter the ovary. Here they find their way to the 

 ovules, which they enter, one pollen-tube going to each 

 ovule and pushing its way through the micropyle, until its 

 end comes in contact with the nucellus and finally with 

 the embryo-sac. A portion of the contents of the pollen- 

 tube, including nuclear material, now passes into the 

 embryo-sac and unites with a cell in it, called the oosphere. 

 The oosphere now takes on a cell-membrane, increases in 

 size, undergoes division, and, as a result of still further 

 division and growth, produces the embryo. Other cells 

 are formed in the embryo-sac which rapidly multiply and 

 become the endosperm, a tissue often absorbed afterwards 

 by the growing embryo prior to germination. Meantime 

 the embryo-sac becomes many times its former size, while 

 the nucellus is crowded to the walls of the ovule and is 

 commonly absorbed, but sometimes remains as the peri- 

 sperm. The coats of the ovule are extended to keep up 

 with this increase in size, the testa takes on its character- 

 istic hard and usually colored condition, a further store of 

 food is deposited around or in the growing embryo, and 

 with the completion of these various processes the ovule 

 has become a mature seed. 



The changes just described, together with some others 

 that chiefly affect the ovary, take place whether pollen 

 from the same flower or from another flower of the same 



