THE PTERIDOPHYTA : LYCOPSIDA, ETC. 567 



Lycopodiales 



The Lycopodiales are distinguished from the FiHcales by the small size 

 of the leaves and bv the fact that the sporangia are aggregated together into 

 cones or strobili, which bear specialized leaves, on the upper surface of 

 which the sporangia develop. These sporangia may be homosporous or 

 heterosporous, and it is on this point that the two important families, the 

 Lycopodiaceae and the Selaginellaceae, are distinguished ; the latter are 

 heterosporous. 



Associated with the small leaves we find that the vascular anatomy is 

 simple compared with that of the Ferns, though several steles may occur 

 in the stem. 



The development of a special structure, the ligule, which appears at the 

 base of the vegetative leaves and also, associated with the sporangia, at the 

 base of the leaves of the cones, is a further point which distinguishes the 

 Selaginellaceae from the Lycopodiaceae. This feature is found very commonly 

 among fossil members of the order, both those which are homosporous and 

 those which are heterosporous. This ligule is a minute scale which arises 

 from a superficial cell of the epidermis of the leaf base. 



The living Lycopodiales are the representatives of a group which, during 

 the Carboniferous period, formed the chief vegetation. Many of the types 

 then growing, such as Lepidoderidron, were large trees comparable in size 

 with the Pines and Firs of the present time. The modern representatives are 

 all quite small plants, seldom reaching more than a foot in height, though 

 some of the trailing species may become longer. 



They occur chiefly in peaty soils, often, in Europe, on the sides of 

 mountains, but in warmer countries large species grow as hanging epiphytes 

 on the trunks of trees and form part of the forest undergrowth. 



^^'e shall consider two examples of this order, Lycopodium davatum and 

 Selaginella kraussiana. 



Lycopodium davatum (The Club Moss) 



Five species oi Lycopodium occur in Britain, and are found most commonly 

 on heaths and moors, chiefly in hilly districts, though they are never very 

 common except in a few localities, where large patches of plants may some- 

 times be found (Fig. 576). 



Lycopodium davatum consists of an extensively branched stem several 

 feet long, which may be partly underground, and is then colourless and 

 leafless, like a rhizome. From it arise secondary-, upright branches which 

 divide dichotomously and may produce terminal strobili. The stem produces 

 adventitious roots, which arise all along the underside in acropetal succession, 

 that is to say, successively towards the apex of the stem. These roots branch 

 dichotomously. The stem above the ground is densely clothed with small 

 leaves about 4 mm. long, which are borne in regular spiral succession, and 

 are all alike in size and shape. 



