LEPIDODENDRIDS 269 



482. Allied to the Club-mosses are the arborescent 

 Lepidodendrids (Order Lepidodendrales) which were 

 abundant in the Paleozoic period, and which disappeared 

 in the Mesozoic. We have fragmentary fossils of the 

 sporophytes, which were large dichotomously branched 

 trees, sometimes 30 meters high and a 



meter in diameter. There was a large 

 central vascular bundle, which how- 

 ever formed a peripheral cambium 

 so that the stems increased their di- 

 ameter much as in the case of higher . ;tJB 

 plants. The stems and branches no. us. Lepidoden- 

 were thickly clothed with pointed ^ a / s s p3 hyte ' 8p r ' 

 leaves a decimeter or more in length, 

 and when these fell off they left large scars of charac- 

 teristic shape and arrangement. 



483. The fossil remains of the spore-bearing cones, of 

 which many specimens have been found, indicate that 

 they contained two kinds of spores. Hence it is certain 

 that the Lepidodendrids were allied to the Club-mosses. 

 The more common genera are Lepidodendron, and 

 Sigillaria. 



Laboratory Studies, (a) Secure a few fresh or alcoholic 

 specimens of various kinds of Lycopods in fruit. Ground 

 Pines may be collected in many places in the eastern United 

 States. The Club-mosses may be obtained in plant-houses. 



(6) Make cross-sections of the stems, and study the vascular 

 bundles in Lycopodium where they are imbedded in a thick 

 mass of fibrous tissue. Examine the leaves, noting the small 

 vascular bundle in the midrib. Study the epidermis, which 

 contains numerous breathing-pores. 



(c) In like manner study Selaginella. 



(d) Carefully remove a sporophyll from a cone of Lycopo- 

 dium, and study the sporangium and spores. Further exami- 

 nation will show that the spores are of one kind only. 



