FUNCTIONS OF THE ORGANS OP THE FLOWER. 13 



always to be found at that end of the ovule which is not 

 attached to the ovary. (Fig. 18, ra.) 



About the time the anthers discharge their pollen, a 

 little cavity, called the embryo-sac, appears inside the 

 nucleus, near the micropyle. The pollen-tube, with its 

 liquid contents, enters the ovary, passes through the 

 micropyle, penetrates the nucleus, and attaches itself to 

 the outer surface of the embryo-sac. Presently the tube 

 becomes empty, and then withers away, and, in the mean- 

 while, a minute body, which in time developes into the 

 embryo, makes its appearance in the embryo-sac, and from 

 that time the ovule may properly be called a seed. 



17. In order that ovules may become seeds, it is always 

 essential that they should be fertilized in the manner just 

 described. If we prevent pollen from reaching the stigma 

 by destroying the stamens, for instance the ovules 

 simply shrivel up and come to nothing. 



Now it is the business of the flower to produce seed, 

 and we have seen that the production of seed depends 

 mainly upon the stamens and the pistil. These organs 

 may consequently be called the essential organs of the 

 flower. As the calyx and corolla do not play any direct 

 part in the production of seed, but only protect the 

 essential organs, and perhaps attract insects, we can under- 

 stand how it is that they, as a rule, disappear early 

 Their work is done when fertilization has been accom- 

 plished. 



Having noticed thus briefly the part played by each 

 set of floral organs, we shall now proceed to the exami- 

 nation of two other plants, with a view to comparing 

 their structure with that of the Buttercup. 



