PRE-CARBONIFBRODB PLANTS. 



19 





wore given off in suflSciont number to cause an irregularity of growth in 

 the layers of wood covering the remains of these branches. This appear- 

 ance may be observed in stems of modern trees giving off many branches 

 at one level. (2.) The old trunks may have produced rings of aerial roots ; 

 perhaps after their bases were partially buried under sediment or accumu- 

 lations of vegetable soil. In some cases holes or depressions occur along 

 the course of the swellings, wliich may mark the points of attachment of 

 the branches or roots referred to (Fig. 19 a). 



That Prototaxites was essentially distinct from any other known tree of 

 the Palaeozoic Period is obvious ; but in the absence of all knowledge of 

 its foliage and fructification, any attempt to divine its affinities must be 

 merely conjectural. Its want of proper vascular tissue, along with its 

 dense woody structure and regular exogenous growth, ally it to conifers ; 

 and among these its spirally marked fibres approach more nearly to those of 

 the Taxineoi than to any other tribe. Among Palaeozoic plants, its structure 

 more nearly resembles that of the wood to which I have given the name of 

 Nematoxylon, than any other type. Indeed this might be placed with 

 Prototaxites, but for the absence of any evidence of exogenous growth in 

 the former. Prototaxites may also be compared with Aporoxylon of linger, 

 but it differs in several essential particulars, though both may be regarded 

 as prototypal conifers. Among more recent fossil species, the tertiary 

 genus Spiropity» of Goeppert presents some distant points of resem- 

 blance.* 



It is perhaps worthy of notice that the plant recently described by Mr. 

 Hincks t {Eophyton explanatum), from the Lower Arenig rocks of St. 

 David's, has a tissue of uniform cylindrical cells resembling those of Pro- 

 totaxites or Nematoxylon. It may have been a root or small branch of a 

 tree of this description. In specimens from the Ludlow of England 

 kindly shown to me by Mr. Etheridge of the Geological Survey of Great 

 Britain , I found fragments of wood with the structure oi Prototaxites. 



Prototaxites is the oldest exogenous plant at present known to us, and 

 the type is as yet confined to the Lower and Middle Devonian. It was 

 contemporary with Dadoxylon in the latter of these periods, but is struc- 

 turally as widely separated from that genus as from modern Taxine and 



• Mr. Carru there has kindly pointed out to rce some structural points in which this remark- 

 able plant resembles Algae of the family Codi», the long tubes traversing which he com- 

 pares with the cells of Frototaxitet. For the reasons stated in the text, however, I cannot 

 accept this as an indication of true afBnity, and must believe the plant to have been a terres- 

 trial tree exogenous in its mode of growth. The high botanical skill of Mr. Carruthers, 

 however, renders it important to state his view?, in the present imperfect state of our know- 

 ledge of this truly wonderful plant. 



t Geological Magazine, Dec, 1869. See also infra, §111. 4. 



