376 PART IV. CLASSIFICATION. 



on each flank. The cavities are surrounded by parenchymatous 

 tissue. 



The globular sporocarp of Pilularia contains four (sometimes two 

 or three) cavities, extending longitudinally from the base to the 

 apex, enclosed by parenchymatous tissue. The placenta is a ridge 

 of tissue on the external wall of the cavity, bearing the sorus which 

 consists, in its upper part, of microsporangia, and in its lower of 

 one or more macrosporangia. The cavities at first communicate 

 with the outer air at the apex of the sporccarp, but eventually be- 

 come completely closed. 



In their development, the sporangia of the Heterosporous Lepto- 

 sporangiatse resemble those of the Homosporous Leptosporangiatse 

 (see p. 363) in all essential points : but no annulus is developed. In 

 each sporangium sixteen spore-mother-cells are developed from the 

 single tetrahedral archesporial cell, and each of these mother-cells 

 undergoes division to form four spores : but whereas in the micro- 

 sporangia all these sixty-four spore-rudiments develope into micro- 

 spores, in the rnacrosporangium only one developes into a 

 macrospore, the others being abortive. 



The development of the spores in this group, is remarkable on 

 account of the important part played by the multinucleate proto- 

 plasmic mass (epiplasm), derived from the disorganisation of the 

 tapetal cells, in which the free spore-mother-cells are embedded at 

 the time when the development of the spores is commencing. 

 Taking first the Salviniacese : the microsporangium of Salvinia 

 contains, when mature, a number of microspores embedded in a 

 spongy mass of a substance, which gives some of the reactions of 

 corky cell-walls and is derived from the protoplasm of the tapetal 

 cells : in Azolla the microspores are likewise embedded in this 

 substance, but in more than one group or massula (2-8) according 

 to the species ; each massula is surrounded by a membrane, bearing 

 in some species a number of anchor-like hairs, the glochidia. In 

 Salvinia the macrospore also is invested by a layer of this spongy 

 substance, constituting the episporc or perinium. This is also the 

 case in Azolla, but here the perinium is remarkably developed : over 

 the rounded dorsal surface of the radial macrospore, the perinium 

 is a thick membrane, firm at the surface, spongy within, with 

 warty projections bearing filaments of the same substance : on the 

 three flattened surfaces of the ventral aspect of the macrospore the 

 perinium forms three or more large spongy masses which constitute 

 the so-called floats of the spore ; at the pointed apex of the spore, 



