410 PART III. THE CLASSIFICATION OF PLANTS. 



of macrosporangia and a double row of microsporangia 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 sporocarp, but 

 eventually become completely closed. 



In their development, the sporangia of the Heterosporous 

 Leptosporangiatae resemble those of the Homosporous Lepto- 

 sporangiatae (see p. 392) 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 microsporangia all these sixty-four spore-rudiments 

 develope into microspores, in the macrosporangium 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 rnassula is surrounded by a 

 membrane, bearing in some species a number of anchor-like hairs, 

 the glochidia (absent in A. pinnata and A. nilotica) : membrane 

 and glochidia are developed whilst the massula still consists of 

 living protoplasm. In Salvinia the macrospore also is invested by 

 a layer of this spongy substance, constituting the epispore 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 



