1 98 MORPHOL OGY. 



gium usually all in that sporangium are of the same kind, so that 

 certain sporangia bear microspores, and others bear macrospores. 

 But it is not uncommon to find both kinds in the same sporan- 

 gium. When a sporangium bears only microspores the number 

 is much greater than when one bears only macrospores. 



400. If we examine some of the microspores of isoetes we see that they are 

 shaped like the quarters of an apple, that is they are of the bilateral type as 

 seen in some of the ferns (asplenium). 



401. Male prothallia. — In isoetes, as in selaginella, the microspores de- 

 velop only male prothallia, and these are very rudimentary, one division of 

 the spore having taken place before the spore is mature, just as in selagi- 

 nella. 



402. Female prothallia. — These are developed from the macrospores. The 

 latter are of the tetrahedral type. The development of the female prothal- 

 lium takes place in much the same way as in selaginella, the entire prothal- 

 lium being enclosed in the macrospore, though the cell divisions take place 

 after it has left the sporangium. When the archegonia begin to develop 

 the macrospore cracks at the three angles and the surface bearing the arche- 

 gonia projects slightly as in selaginella. 



403. Embryo. — The embryo lies well immersed in the tissue of the pro- 

 thallium, though there is no suspensor developed as in selaginella. 



