SEXUAL REPRODUCTION. 299 



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

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

 (fig. 342) and are shaken out. 



405. In angiosperms the development of the embryo 

 stimulates the belated female plant to complete its growth, 

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

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

 pines. 



406. Endosperm. — The growing endosperm and the 

 embryo sporophyte, which it surrounds, crowd upon the 

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

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

 the embryo continues to grow, it may crowd upon the endo- 

 sperm until 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 sporophyte as a storehouse of food for the use of 

 the embryo sporophyte when it resumes 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 perispertn. 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: (1) in the interior, occupying 

 various positions and of exceedingly variable relative size, the 

 embryo; (2) immediately around 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- 



