THE VEGETABLE KINGDOM 335 



moss-like Lycopodium and Selaginella) the Equisetales (by 

 the marsh-loving " horse-tails ") and the Filicales (ferns). 

 The two first, at least, existed in the earliest known land 

 flora (Devonian) and attained their highest development 

 in the newer Palaeozoic, and alongside them are found 

 both undoubted ferns and another group, Sphenophyllales, 

 which appears to represent the common stock from which 

 they sprang. The extensive working of coal in the 

 Carboniferous rocks has enabled the flora to be collected 

 very extensively and studied very thoroughly, and some 

 explanation is necessary of the conditions of preservation 

 of these fossils. 



The remains of land-plants are of two kinds impressions 

 of leaves, bark, etc., which show in a carbonaceous film 

 the outward form and surface- markings, often very 

 perfectly, but preserve no internal structure ; and petrifac- 

 tions, either in silica or calcite, which preserve internal 

 structure often as perfectly as in a recent plant kept in 

 methylated spirit, but may often fail to show outward 

 form. Further, the various parts of a complete plant 

 root, stem, leaves, sporophylls, seeds are rarely found 

 in organic continuity, and even such a single part as the 

 stem may present very different appearances according 

 as the outer cortex is preserved or not, etc. It is evident 

 therefore that the palaeobotanist has to reconstruct his 

 plants out of very scattered and fragmentary evidence. 

 Great difficulties of nomenclature arise : leaf impressions 

 receive one set of names, stems another, roots another, 

 and even when leaf, stem, and root have been pieced 

 together it may be found that leaves whose forms differ 



