SEXUAL KEPRODUCTION. 299 



When firm at maturity the tone scales open on drying, and 

 the seeds, each with a wing attached, split off from the scale 

 (fig. 342) and ai-e shaken out. 



405. In angiosperms the de\elopment of the embryo 

 stimulates the belated female plant to complete its growth, 

 and the megaspore (embryo-sa( ) is soon entirely filled by it. 

 This late-forming gametophyle is called endosperm, as in the 

 pines. 



406. Endosperm. — The growing endosperm and the 

 embryo sporoi)hyte, which it surrounds, crowd upon the 

 sporangiuui. This may, therefore, partly or wholly dis- 

 appear. If, when the full size of the endosperm is reached, 

 the embr^ o continues to grow, it may crowd upon the endo- 

 sperm imtil a part or all of it is absorbed. The embryo 

 sooner or later passes into a resting stage and ceases to en- 

 large. In this dormant condition it remains for a time whose 

 duration is chiefly determined by external conditions. 



407. Food. — The tissue of the endosperm is utilized by 

 the parent spjorophyte as a storehouse of food for the use of 

 the embryo s]joroi)hyte when it resiuiies growth. If the 

 embryo displaces the endosperm, it absorbs the reserve food 

 therein, consisting of starch, oil, or aleurone grains (^ 236). 

 In case any tissue belonging to the sporangium remains, this 

 also is utilized for storage. To distinguish it from the endo- 

 sperm it is called perisperui. It is only occasionally present 

 in any amount in this group of plants. 



408. The integuments of the ovule at the same time en- 

 large, and finally become differentiated in such fashion as to 

 constitute the seed-coats. The ripened seed, therefore, con- 

 sists of the following parts: (ij in the interior, occup)-ing 

 various positions and of exceedingly variable relative size, the 

 embryo; (2) immediately aroimd this, the endosperm or peri- 

 sperm, or both ; but either or both may be so shrunken 

 and emptied as to be recognizable only by microscopic ex- 



