210 BOTANY. 
velop. After fertilization the germ-cell gives rise directly 
to a leafy plant, which emerges from the spore-wall in a 
way to remind one very forcibly of the growth of a plantlet 
from a seed. This resemblance is made greater by the 
likeness the first leaves bear to cotyledons (Fig. 119). 
But one genus, Selaginella, is known in this order. It contains 
about three hundred species, most of which are tropical. Two only 
(viz., 8. rupestris and §. apus) are common throughout the United 
States, although five others are indigenous. Several exotic species 
are commonly cultivated in plant-houses, 
441. The Quillworts (Order Isoétacew) are small grass- 
like plants, with narrow leaves growing from short, thick, 
tuber-like stems. They grow in water or muddy places. 
442. Their spores, which are of two kinds, are produced 
in spore-cases on the upper surfaces of the leaf-bases, In 
their germination and development of the sexual organs 
they resemble the plants of the previous order. 
The Quillworts are all of one genus, Isoétes, of which 
there are in the United States fourteen species. 
Fossil Lycopods.—Two orders of Lycopods once existed, containing 
large trees, which appear to have been very abundant. The Lepido- 
dendrids (Order Lepidodendracee were a metre (2 to 4 feet) thick 
and 15 to 20 metres (45 to 60 feet) high, and seem to have had the 
general appearance of the Club-mosses. The Sigillarids (Order Sigil- 
lariacege) appear to have been trees 30 or more metres (100 feet) in 
height and 14 metres (4 or 5 feet) in diameter. Both produced two 
kinds of spores, showing their relationship to the Little Club-mosses 
and the Quillworts. Although very abundant in the Coal Period, 
they have long since become entirely extinct. 
Practical Studies.—(a) Secure a few fresh or alcoholic specimens 
of various kinds of Lycopods in fruit. The Little Club-mosses may 
be readily obtained in plant-houses. Make cross-sections of the stems 
and study the fibro-vascular bundles, which in Lycopodium are im- 
bedded in a thick mass of fibrous tissue. Examine the leaves, noting 
