552 



INTRODUCTION TO CRYPTOGAMIC BOTANY. 



a metamorphosed leaf or footstalk divided by septa, or unilo- 

 cular ; antheridia in the same secondary receptacle with the 

 monosporous sporangia, or in a distinct sac ; prothallus con- 

 fluent with the spore, not forming a distinct expansion. 



631. The plants of which this curious natural order is com- 

 posed are all aquatic, and are so diversified in structure, though 

 they have several characters in common, that they may be, 

 and indeed have been, referred to two distinct natural orders. 

 Marsilea and Pilularia, for instance, are associated in one 

 group, Salvinia and Azolla in another. Isoetes, though 



a. Pilularia minuta, Durieu, 

 natural size, from a cultivated 

 specimen. 



b. Eeceptacleof ditto, slightly 

 magnified. 



Fig. 120. 



Eeoeptacle of P. minuta, di- 

 vided vertically, so as to show a 

 sporangium and three anthe- 

 ridia. Prom Expl. Scient. de 

 I'Algerie. 



slightly anomalous, is clearly a Lycopod. The four first 

 agree in having receptacles formed, evidently, from a meta- 

 morphosed leaf or leafstalk, while in Isoetes the fruit is 

 lodged in the axil of the leaf, and probably arises, not from 

 the leaf itself, but from an axillary bud. Its spores and male 

 organs are almost identical with those of LycopocUa. 



632. The affinities of these plants with other Filicals are at 

 first far from evident, the structure of the fruit is so very 



Cryptogams sporangia, according to the commonly received nomencla- 

 ture. But as homologous organs should have the same name, the word 

 sporangia is here used for the ultimate common envelope of the spores. 



