VIIl] DETERMINATION OF MOSSES. 237 



definite central cylinder consisting in the centre of elongated 

 elements with dark- coloured and thick walls having thin 

 transverse septa; surrounding this central tissue there are 

 thinner walled elements, of which some closely agree in form 

 with the sieve-tubes of the higher plants. ' The central tissue 

 may be regarded as a rudimentary type of xylem, and the 

 svirrounding tissue as a rudimentary phloem. Each leaf is 

 traversed by a median conducting strand which passes into the 

 stem and eventually becomes connected with the axial cylinder. 



The fertilisation of the egg-cell gives rise to the development 

 of a long slender stalk terminating distally in a large spore- 

 capsule. In section the stalk or seta closely resembles the 

 leafy axis of the moss plant. Considering the fairly close 

 approach of some of the mosses to the higher plants as regards 

 histological characters, it is conceivable that imperfectly petri- 

 fied stems of fossil mosses might be mistaken for twigs of 

 Vascular Cryptogams. 



Like Liverworts, Mosses have left very few traces of their 

 existence in plant-bearing rocks. Without the aid of the 

 characteristic moss-' fruit ' or sporogonium it is almost impos- 

 sible to recognise fossil moss-plant fragments. In species of 

 the tropical genera Spiridens and Dawsonia, e.g. 8. longi- 

 folius^ Lind. or D. superba^ Grev. and B. polytrichoides^ R. Br., 

 the plant reaches a considerable length, and resembles twigs of 

 plants higher in the scale than the Bryophytes. The finer 

 branches of species of the extinct genus Lepidodendron are 

 extremely moss-like in appearance. Again, Cyathophyllum 

 bulbosum Muell *, with its two kinds of leaves arranged in rows, 

 is not at all unlike species of Selaginella or the hepatic 

 genus Gottschea. It is by no means improbable that some of 

 the Palaeozoic specimens described as twigs of Lycopodites, 

 Selaginites, or Lepidodendron, may be portions of mosses. 

 The fertile branches of Lycopodium phlegmaria in a fossil 

 condition might be easily mistaken for fragments of a moss. 

 In some conifers with small and crowded scale-leaves there 

 is a certain resemblance to the stouter forms of moss stems. 



1 Sohimper (65) PI. in. " Greville (47) PI. xii. 



» Brown, E. (11) PI. xxiii. ^ Hooker, W. J. (20) PI. CLxn. 



