- 392 Structural and Physiological Botany. - ; 
hyemale being on this account especially used asa substance for polish- 
ing, under the name of ‘ Dutch rushes.’ /. arvense isa troublesome § ~ 
weed. 
Crass IX. Ophioglossacee. 
The.plants belonging to this class were till recently in- 
cluded among Ferns ; but are sufficiently distinguished from 
them by their underground pro-embryo, which is destitute 
of chlorophyll ; by the mode of development and the form 
of the sporangia ; and finally, by the stem never branching. 
The primary root seldom branches ; and the small, flat stem is 
enclosed by the sheathing base of the leaf. The number of 
leaves, which are always annual, is small; our two native | 
species, Ophioglossum yulgatum (Fig. 446), and Botrychium 
Lunaria, never producing more than one each year. The 
leaves are also remarkable for the slowness of their growth ; 
those of the latter species requiring four years for their de- 
velopment, the first three of these being spent entirely be- 
neath the soil. As soon as the plant has attained a certain 
age, the leaf branches, one half developing into a receptacle 
or spike of sporangia, [the other into the green lamina of the 
leaf|. The sporangia are not produced each from a single 
epidermal cell, and therefore of the nature of trichomes, as 
in Ferns ; but are rather entire lobes of the leaf, the internal 
tissue of which is the origin of the mother-cells of the 
spores. In Od/zoglossum the sporangia are arranged in two 
rows ina simple spike (Fig. 446 1., 11.), and are coherent ; 
in Botrychium, on the contrary, they are distinct, and form 
a branched spike or panicle. The tetrahedral spores pro- 
duce, on germination, prothallia, on which are developed 
the reproductive organs, antheridia and archegonia ; but 
the process of impregnation and the first stages of develop- 
ment of the oospore have not yet been observed. ‘There is — 
the same alternation between a sexual generation producing 
antheridia and archegonia, and a non-sexual generation pro- ~ 
ducing spores, as in the last two classes. In Ophzoglossum. — 
