8a CONIFERAE. 



greater or less abundance. Medullary rays simple. 4. Pityoxylon. Tra- 

 cheides as in Cupressoxylon. Resin-passages besides the parenchymatous 

 wood-cells, but still surrounded by secretion-tissue. Medullary rays of two 

 kinds ; those in several rows with a resin-passage running horizontally through 

 their centre. 5. Cedroxylon. Exactly as Cupressoxylon, only without resin- 

 iferous wood-parenchyma. 6. Taxoxylon. Distinguished from Cedroxylon 

 merely by the well-known spiral striation which projects on the inner 

 surface of the wall of the tracheides, but which must not be confounded 

 with the annular and spiral striation in the substance of the membrane 

 common in the autumn-formed tracheides of coniferous woods. 



We might think that it would be easy to discover and apply the above 

 differential characters in fossil woods, but the task is not unaccompanied 

 with difficulties. For instance, it is not always quite easy to separate the 

 wood of the roots of Cupressoxylon and Cedroxylon with pits in several 

 rows (Eleoxylon, Brongn. 1 ) from Araucaroxylon, and the distinguishing the 

 Araucaroxyla with pits in a single row from the other two groups may have 

 its perplexities ; and hence we see Pinites latiporosus, Cramer 2 , referred by 

 Kraus 3 to Araucaroxylon, and by Schroter 4 to Cedroxylon. Again, the 

 difference between Cupressoxylon and Cedroxylon is obscured by the 

 circumstance that in some Cupressineae the wood contains an unusually 

 small number of resiniferous cells, of which Beust gives many examples 5 . 

 Individual as well as specific variations have their effect, so that we can 

 scarcely feel quite sure that there are no such variations to be taken into 

 account. It is also easy to mistake tracheides anomalously filled with 

 resin for parenchymatous wood-cells, and difficult to distinguish the latter 

 in the sections when, as often happens, they contain no resin ; nor must all 

 brown substances filling the cells in fossil woods be supposed to be resin 

 without careful examination ; the presence or absence of the excretion in 

 the wood of Ginkgo, for example, is still in dispute, as will appear by com- 

 paring the statements of Kraus and Bcust. 



Attempts have lately been made to establish further distinctions 

 between fossil woods founded on the characters of the medullary rays. 

 Kraus 6 had already used these characters to break up Pityoxylon, and the 

 process was thoroughly carried out in this group by Schroter 7 . From want 

 of personal experience as regards the constancy of all these characters I am 

 not in a position to criticise the results of these efforts, and I must refer the 

 reader to the original literature. At the same time I would point out that 

 all of them, not excepting Kleeberg's 8 which is the most recent, suffer from 

 a fault in method which ought to have been avoided after the appearance of 

 von Mohl's and Kraus' publications ; the observers have pursued their investi- 



1 See Conwentz (1) and (2). * Heer (5), vol. i, t. 40. s Schimper (1). 4 Heer (5), 



vol. 6 IV, p. 9. See also Felix (1), p. 4, upon Rhizocedroxylon Hoheneggeri. 5 Beust (1). 



6 Schimper (1). 7 Heer (5), vol. 6 iv. Kleeberg (1). 



