Family Lycopodiaceæ.—There are two genera. Lycopodium (club moss) includes many species, most of them tropical, but a number in temperate and subarctic regions. The gametophyte of many species is tuberous, lacks chlorophyll, and in some there lives an endophytic fungus. Phylloglossum with one species is found in Australia.
Family Psilotaceæ.—There are two genera. Psilotum chiefly in the tropics has one species (P. triquetrum) in the region of Florida.
Family Selaginellaceæ.—These include the little club mosses, with one genus, Selaginella (see [Chapter XXX]).
CLASS ISOETINEÆ.
606. Order Isoetales, with one family Isoetaceæ and one genus Isoetes (see [Chapter XXXI]). There are about fifty species, with about sixteen in the United States.
[CHAPTER XXXIII.]
GYMNOSPERMS.
The white pine.
607. General aspect of the white pine.—The white pine (Pinus strobus) is found in the Eastern United States. In favorable situations in the forest it reaches a height of about 50 meters (about 160 feet), and the trunk a diameter of over 1 meter. In well-formed trees the trunk is straight and towering; the branches where the sunlight has access and the trees are not crowded, or are young, reaching out in graceful arms, form a pyramidal outline to the tree. In old and dense forests the lower branches, because of lack of sunlight, have died away, leaving tall, bare trunks for a considerable height.