L VCOPOnTNEAE.—UGUr.A TAE. 



=93 



released from the sporangium, in the position assigned them by the division of the 

 mother-cell at the four corners of the tetrahedron. It is not uncommon to find weakly 

 macrospores in otherwise normal spikes of sporangia. The three layers of cells 



Fig. 237. Sporangia and development of the spores of Selaginella itiaeqitalifolia. The order of succession is 

 according to the letters A—D. A and B represent stages in all sporangia. C and D represent stances in the micro- 

 sporangia. E development of the spores ; e—h marks the succession, h four nearly ripe spores. In A. C, and B a and 

 * are the two-layered wall of the sporangium, e the tapetal cells, d the mass of sporogenous cells. A and A' magn. 

 500, C and D 200 times. 



which form the wall of the sporangium remain intact till the spores are ripe, while 

 the tapetal cells are destroyed during their formation, as happens in the Ferns. 



The sporangia of Isoetes are formed in the fovea of the leaf-sheath and are 

 attached by a narrow base (Fig. 241). In this case they are undoubtedly the product 

 of the leaf. The outer leaves of the fertile rosette produce only macrosporangia, the 

 inner only microsporangia, and the former contain a large number of macrospores. 

 Both kinds of sporangia are imperfectly divided into compartments by threads of 

 tissue {trabectdae) that stretch across from the ventral to the dorsal side ; they do not 

 open, but the spores are set free by the decay of the wall. 



In Isoetes as in Selaginella the course of development is the same up to a certain 

 point in both kinds of sporangia, but that point is sooner reached in Isoetes. The 

 sporangia originate in a group of cells at the bas* of the leaf, but are much larger than 

 in Selaginella. The archesporium is a hypodermal layer of cells. In the micro- 

 sporangia the cells of the archesporium elongate and divide by transverse walls. In 

 this condition no difference is yet to be seen between the sterile rows of cells (the 



