THE ISOETACE^. 



213 



The sporangia in the outermost leaves contain numer- 

 ous large spores called megaspores, macrospores, or 

 gynospores, while nearer the centre of the plant similar 

 sporangia bear a much larger number of smaller spores 

 called microspores or androspores. The megaspores 

 always produce female prothallia, and the microspores 

 those of the opposite sex. The megaspores are about 

 one fourth the size of a pin's head, but are large enough 

 to be seen with the unaided e)'e. Under a lens strong 

 enough to resolve their structure they appear as spheres 

 of dazzling white encircled by an elevated ridge near the 

 middle, called the equator, and with three other ridges 

 called commissures going from the equator to meet in 

 the centre of the upper hemisphere. One half of the 

 spore is thus divided into 

 three nearly equal areas, 

 while the other is with- 

 out ridges. Between the 

 ridges, especially on the 

 upper hemispliere, the 

 surface is thrown into 

 many curious patterns in 



1 • 1 ■ . -, Megaspores. (Greatly enlartred.) 



which spines, crests, pits, '' ^ ' '^ ' 



and labyrinthine forms predominate. Each separate 

 species has its own pattern of marking, and thus the spores 

 afford important characters for identification. The. white 

 coating of the spores has been found by Mr. Raynal 

 Dodge to be silica, the same flinty matter that gives 

 roughness to the stems of the scouring-rush. 



The microspores are about one thousandth of an inch 

 in length and very numerous. According to Dodge 

 there may be as many as a million spores in a single 

 sporangium. They are usually oblong, somewhat triangu- 



