1922] 



PFEIFFER MONOGRAPH OF THE ISOETACEAE 89 



The group of microspores from a mother cell may show bi- 

 lateral or tetrahedral arrangement. In either event, the freeing 

 of the spores soon allows the loss of the sharpness of the original 

 form, so that the microspore is more or less rounded at the two 

 side angles, though with a sharper angle on the third side, paral- 

 lel to the long axis. Usually the long diameter of the microspores 

 will fall between 20 and 40 n, with the width or thickness less 

 than 25 n. In some forms, there are markings of the exospore, 

 resulting in papillose or spinulose effects. More rarely, there may 

 be the development of a wing extension or crest, especially on 

 the side opposite the sharp angle. There is little variation in 

 coloring in the microspores, which are generally ashy, fawn, or 

 cinnamon-brown. As diagnostic features, the microspore size, 

 primarily, and markings and coloring, secondarily, may be used 

 to advantage. 



The study of the megaspores has many advantages over that 

 of microspores. Their greater size, due to the functioning of 

 fewer cells in the sporangium at the expense of other potentially 

 sporogenous cells, makes for facility in handling. The size varies 

 in the species between 250 and 900 n in general, with a much 

 smaller range in the individual species. The coloring of the 

 megaspores in most cases is white or gray-white but in cases of 

 exceptions, as the black in /. melanospora Engelm., the color is 

 a useful diagnostic character. Most important are the markings 

 or sculpturing on the surface of the siliceous exospore. The 

 spores, formed in tetrahedral groups, retain the impression of 

 their three neighbors on three faces, which are separated by 

 rounded or sharp ridges which converge at the apex (sometimes 

 called commissural ridges). The fourth face of each spore, ori- 

 ginally the free surface in the tetrad, is hemispherical or but 

 slightly flattened, and is separated from the three more nearly 

 plane faces by the ridge called the "equator." 



The sculpturing, consisting of spines, of small tubercles or 

 warts, or of reticulate markings, may be similar on all faces, or 

 may differ from the other three on the fourth or basal side. 

 These characters within the species seem as conservative features 

 as occur in Isoetes and therefore of great value diagnostically. 



The sporangia in Isoetes lack a definite device for dehiscence, 

 and hence both types of spores are released only upon decay of 

 the sporangium walls. As a result, there is an accumulation of 



