158 THE EVOLUTION OF PLANTS 



nate bands or becoming intermingled in other 

 ways. It is a point of interest that this pecul- 

 iar structure often extends to the roots. Each 

 leaf is supplied by a single unbranched vascular 

 strand, a simple arrangement which prevails 

 throughout the whole class, so far as its recent 

 members are concerned. In some of the exotic 

 species of Lycopodium the stem is very robust 

 and the leaves of considerable breadth, but the 

 general structure varies but little throughout 

 the genus. The forking of the stem is the char- 

 acteristic mode of branching. 



The sporangium is a comparatively large sac, 

 conspicuous to the naked eye, attached to the 

 base of the fertile leaf on its upper side. It pro- 

 duces an immense number of minute spores 

 ("Lycopodium Powder," used in fireworks). 

 It is only in recent years that the germination 

 of these spores and the prothalli produced from 

 them have become known. One difficulty is 

 that the spores in many cases require to rest an 

 extraordinary time before they will begin to 

 germinate from three to five years in Lycopo- 

 dium Selago and from six to seven years in the 

 Stag's Horn Moss (L. clavatum). Even then the 

 development is astonishingly slow, so that in 

 L. Selago it takes from six to eight years and 

 in L. clavatum from twelve to fifteen years after 

 the sowing, before the prothalli are mature 



