160 THE EVOLUTION OF PLANTS 



leaves and a little cone. The prothallus is green, 

 like that of a Lycopodium of the L. inundatum 

 type. The genus is interesting, because the 

 mature plant much resembles the embryonic 

 stage of some species of Lycopodium, but whether 

 this means that Phylloglossum is a primitive or a 

 reduced form is still an open question. 



The family Lycopodiaceae is a curious and 

 isolated group, which we may reasonably sup- 

 pose to represent an ancient race. Unfortunately 

 next to nothing is known of its geological history. 

 An Upper Triassic plant, Naiadita, has much in 

 common with Lycopodium. At present, how- 

 ever, there is no case of a fossil Lycopod in which 

 we can be certain that the spores were all of one 

 kind. Whenever there is definite evidence, the 

 fossil Club-mosses have proved to have spores 

 of two kinds. It is difficult to prove from fossil 

 specimens that only one kind of spore existed, 

 for we cannot distinguish between microspores 

 and the uniform spores of a plant with one kind 

 only, whereas the opposite condition is at once 

 demonstrated as soon as a megaspore is detected. 



We now go on to the heterosporous Lycopods, 

 the Selaginellacese, a much more satisfactory 

 group from an evolutionary point of view. The 

 two genera, Selaginella and Isoetes, are extremely 

 different, and probably not at all closely allied. 



