416 BOTANY 



terminal cone-like flowers. Compared with the leaves, the sporangia 

 are relatively large. They are developed in the same way as those of 

 the eusporangiate Filices and Equisetinae, from a projecting group of 

 cells derived from the epidermis and the underlying tissue ; while in 

 all other Fteridophyta the sporangia are developed from a single 

 epidermal cell. The innermost layer of the sporangial wall, the 

 tapetal layer, is absorbed. The sporangia have no annulus. Except 

 in the case of Isoetes, whose spores become free by the decay of the 

 sporangial wall, they dehisce by longitudinal slits, which divide the 

 sporangia into two or more valves ; the slits occur where the walls of 

 rows of cells have remained thin. The sporangia of Lycopodium are 

 homosporous ; those of other Lycopodinae heterosporous. The hetero- 

 sporous forms produce only greatly modified and reduced prothallia ; 

 in the genus Lycopodium, on the other hand, the prothallia are essen- 

 tially the same as those of the Filices and Equisetaceae. In the develop- 

 ment of their asexual generation the heterosporous Lycopodinae resemble 

 in many respects the heterosporous Hydropterideae. 



The dichotomous branching of the stems and roots is characteristic 

 of this class (Figs. 18, 19, p. 19); in the genus Isoetes, however, the 

 stem is not only unbranched but also tuberous. 



Order 1. Lyeopodiaceae 



The numerous, widely distributed species of the genus Lycopodium 

 (Club Moss) are for the most part terrestrial plants; in the tropics 

 epiphytic forms also occur. In Lycopodium clavatum, one of the 

 commonest species, the stem, which is thickly covered with small, awl- 

 shaped leaves, creeps along the ground ; it branches dichotomously, 

 and gives rise to ascending lateral branches, while from the under 

 side spring the dichotomously branched roots (Fig. 349). The flower- 

 cones, consisting of the closely aggregated sporophylls, are situated in 

 groups of two or more at the ends of the forked, erect shoots. The 

 sporophylls are not like the sterile leaves in shape ; they are broader 

 and more prolonged at the tip ; each bears a large reniform sporangium 

 on the upper side at the base. The sporangium opens into two valves 

 by a transverse slit, and lets free numerous minute spores (Fig. 

 349, 2). 



Lycopodium Selago differs in habit from the other species ; its 

 bifurcately-branched stems are all erect, and the flower-cones are not 

 distinct from the vegetative region of the fertile shoots. 



The spores of the Lycopodiums are all of one kind, and in conse- 

 quence of their formation in tetrads are of a tetrahedral though some- 

 what rounded shape. The exine is covered with a reticulate thickening. 



The mode of germination and development of the sexual generation 

 have as yet been determined only for a few species. The prothallia of 

 Lycopodium annotinum, a species nearly related to L. clavatum, were 



