=590. Sporangia of isoetes.=—If we pull off some of the leaves of the plant we see that they are somewhat spoon-shaped as in fig. 337. In the inner surface of the expanded base we note a circular depression which seems to be of a different texture from the other portions of the leaf. This is a _sporangium_. Beside the spores on the inside of the sporangium, there are strands of sterile tissue which extend across the cavity. This is peculiar to isoetes of all the members of the class of plants to which the ferns belong, but it will be remembered that sterile strands of tissue are found in some of the liverworts in the form of elaters. [Illustration: Fig. 337. Base of leaf of isoetes, showing sporangium with macrospores. (Isoetes engelmannii.)] [Illustration: Fig. 338. Section of plant of Isoetes engelmannii, showing cup-shaped stem, and longitudinal sections of the sporangia in the thickened bases of the leaves.] =591.= The spores of isoetes are of two kinds, small ones (microspores) and large ones (macrospores), so that in this respect it agrees with selaginella, though it is so very different in other respects. When one kind of spore is borne in a sporangium 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 sporangium. When a sporangium bears only microspores the number is much greater than when one bears only macrospores. =592.= 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). =593. Male prothallia.=—In isoetes, as in selaginella, the microspores develop only male prothallia, and these are very rudimentary, one division of the spore having taken place before the spore is mature, just as in selaginella. =594. Female prothallia.=—These are developed from the macrospores. The latter are of the tetrahedral type. The development of the female prothallium takes place in much the same way as in selaginella, the entire prothallium 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 archegonia projects slightly as in selaginella. Absorbing organs in the form of rhizoids are very rarely formed. =595. Embryo.=—The embryo lies well immersed in the tissue of the prothallium, though there is no suspensor developed as in selaginella. CHAPTER XXXII. COMPARISON OF FERNS AND THEIR RELATIVES.