234 BOTANY. 



435. The large spore likewise produces a very small 

 sexual plant, which in this case, however, protrudes a little 



from the ruptured spore-wall. 

 Upon this several archegones 

 develop. After fertilization the 

 germ-cell gives rise directly to a 

 leafy plant, which emerges from 

 the spore-wall in a way to re- 

 mind one very forcibly of the 

 growth of a plantlet from a seed. 



Fig. 140.— Plantletsof aLittle „, . ,, . . 



Club-moss cseiagineiia mar- i his resemblance IS made greater 



tensli), showing cotyledons. I, 



two piantiets growing from one by the liKcness of the first leaves 



spore ; p, tlie first stage (pro- ■' 



thallium). IT, a plantlet sepa- to COtyledonS (Fig. 140). 



rated from the spore ; r, root; /, •' V & / 



Mlgnifl^d™ '^ ■* "*°°'"" But one genus, Selaginella (Family 



Selagmellacew) is known in tbis order. 

 It contains 384 species, most of which are tropical. Two only 

 (viz , S. rupestris and S. apus) are common throughout the United 

 States, although six others are indigenous. Several exotic species 

 are commonly cultivated in plant-houses. 



436. The ftuillworts are all of one genus, Isoetes, of 

 which there are seventeen species in the United States. 



Many botanists now hold that the Quillworts are more 

 nearly related to the ferns {Filicinm) than to the Lycopods, 

 and it is probable that this view may eventually be 

 accepted. 



Fossil Lycopods. — Two orders of Lycopods once existed, containing 

 large trees, which appear to have been very abundant. The Lepido- 

 dendrids (Order Lepidodendraceae) were a metre (3 to 4 feet) thick 

 and 15 to 20 metres (45 to 60 feet) bigh, and seem to have had the 

 general appearance of the Club-mosses. The Sigillarids (Order 

 •Sigillariaceje) appear to have been trees 30 or more metres (100 feet) 

 in height andl^ metres (4 to 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. 



