324 = Structural and Physiological Botany... = " 
form of tissue is parenchymatous fundamental tissue, con- 
sisting of long, nearly cylindrical cells, thin-walled and full 
of sap, and with large intercellular spaces. Both sides of the 
leaves are covered by an epidermis provided with numerous 
stomata. The vascular bundles form a network in the stem, 
the tissue which occupies the space between the meshes 
being frequently changed into scalariform vessels, so that 
the stem then forms a closed hollow cylinder. ‘This often © 
takes place only on one side. 
[The adder’s-tongue, Ophioglossum vulgatum, and the moonwort, 
Botrychium Lunaria, are the only British representatives of the class, 
which comprises only a very small number of species. None are of any 
economical value. | 
Cuass X. LRhizocarpee. 
The Rhizocarpez are plants floating on the surface or 
creeping along the bottom of water, and derive their name 
from the circumstance that their fructification is placed at 
the base of the leaves or between the fibres of the roots. 
From their mode of life they have also been called ‘ Hydro- 
pterides’ or Water-ferns. The stem has a simple vascular 
bundle in its axis, which branches with the stem and also 
passes into the leaves, and is surrounded by an annular 
layer of lignified cells and a parenchymatous cortex. ‘The 
woody part (xylem) of the bundle consists of very thick- 
walled vascular cells, among which occurs here and there an 
elongated parenchymatous cell, while the bast-portion con- 
tains parenchyma, vessels, and bast-fbres. The root has 
also a central bundle, which is, however, surrounded by a 
sheath. The fructification consists of conceptacles or 
sporocarps, containing a number of sforangia collected in 
groups or sovz. The spores themselves are of two kinds: — 
macrospores, and mtcrospores several hundred times smaller. 
They are formed in different sporangia, macrosporangia and 
microsporangia ; the two kinds being found either in the 
