204 MORPHOLOGY. 



developed, which, taking the form of the embryo, one end finally 

 breaks through nuclear papilla of the seed-bud, which up to 

 that time appeared as a thin-walled sac, and now assumes the form 

 of a round sheath (Pilularia), or a flattened, bilabiate body (Sal- 

 vinia). In Salvinia the embryo, when it emerges, produces a 

 pedicle which expands above into a flat discoid body, swimming on 

 the water (first leaf, cotyledon), from the point of attachment of 

 which, below a vertical slit in the same, a bud previously existing 

 in a rudimentary condition develops into a little stem clothed with 

 leaves on both sides, and sending out adventitious roots below. 

 In Pilularia the projecting end of the embryo develops into an 

 upright filament of a green colour (first leaf, cotyledon), at the base 

 of which a pre-existing bud develops into a stem with long fila- 

 mentous leaves. The part of the embryo opposite to the protrud- 

 ing end becomes the root, and breaks through, at a later period, 

 the green papilla of the seed-bud, which also then appears like 

 a sheath. Full-grown plants of Pilularia and Marsilea grow 

 in boggy ground. Their slender stem runs horizontally forward 

 with elongated internodes, producing at the side, and always some- 

 what to the under side of the clavate expanded apex, leaves which 

 in Pilularia are filiform, in Marsilea consist of a long stalk (petiolus) 

 and a four-lobed blade (lamina) ; on the under side the stem con- 

 stantly shoots forth adventitious roots, branches by the develop- 

 ment of axillary buds, and apparently also by a bifurcating division 

 of the apex of the stem. Salvinia, on the other hand, floats freely 

 on the surface of the water; its equally slender stem, with short 

 internodes, bears on both sides shortly-stalked, flat, ovate leaves, 

 sends down adventitious roots in the water from the fruit-stalks, 

 and ramifies little by development of axillary buds. Azolla, a 

 tropical genus, resembles a delicate floating Liverwort. Its course 

 of development is as yet wholly unknown. 



When in the year 1837*, in my survey of the history of development, 

 I observed that I believed that, with respect to the Rhizocarpece in par- 

 ticular, much yet remained to be investigated, I had three points in view ; 

 first, the peculiar formation of the reproductive organs, described indeed 

 by many, but by no one properly understood ; secondly, the inconceivable 

 imperfection of all preceding accounts of germination ; and, thirdly, an 

 isolated observation on Salvinia. In reference to the first point, the 

 essential resemblance of the so-called large spore to the seed-bud, and of 

 the smaller to the pollen-grain of the Phanerogamia, appeared especially 

 remarkable. Touching the second point, it struck me that the germi- 

 nation indicated either the simple development of a plant already per- 

 fectly organised in a rudimentary condition (in the Phanerogamia), or 

 the development of a single cell into a new plant (in the Cryptogamid) ; 

 but that in the treatises upon the germination of the Rkizocarpece there 

 had been no idea either of demonstrating the existence of an embryo 

 capable of development, or of tracing a single spore-like cell in its 



* Wiegmann's Archiv, Jahrg. 1837, vol. i. p. 316. Schleiden's Botanischcn Beitrag. 

 vol. i. 



