58 DESCRIPTIVE CATALOGUE 



in general, and revises the values attached to several features. 

 He points out the importance of comparing parts of similar age, 

 owing to the changes which take place as the si/e of the plant 

 increases ; and notes many other precautions which have been 

 largely overlooked by the older writers. He carefully considers 

 the values of the tangential pits in the tracheids, draws up 

 general rules regarding their occurrence, and gives careful 

 measurements and comparisons for many details of wood- 

 structure. Regarding the medullary rays he says : " Es itst 

 offenbar, dass von den beiden Merkmalen, der Haufigkeit und 

 der Hohe der Markstrahlen, das erstere bessere diagnostische 

 Verwerthbarkeit verspricht als das letztere ; die Haufigkeit der 

 Markstrahlen diirfte daher zweckmiissig als regelnuissiger 

 Terminus in jede Diagnose einzufiihrcn sein." But later work 

 has undermined the value of this feature also, though the 

 importance of the medullary rays is now more fully recognised 

 than ever before. 



In many of the older diagnoses and descriptions, the vertical 

 height of the medullary rays is held to be a character of specific 

 value; the exhaustive and detailed work of Essner (1883), 

 accompanied by many tables of measurements, disposes of this 

 view, for he concludes that " Endlich zeigen aiich verschiedene 

 Individueri derselben Art nicht nur in den gleichen Jahrringen, 

 sondern auch in Gesammtheit betrachtliche Differenzen." 



Beust (1884) and Vater (1884) both consider at length the 

 determination and estimation of the values of various details 

 in fossil woods in papers which are still often referred to. 

 An early and careful account of the pitting in the medullary ray- 

 cells is given by Kleeberg (1885), who also examined the 

 character of the walls in the ray-tracheids. 



A useful historical summary of the numerous papers published 

 up to date appears in Knowlton's (1889 A) work on the fossil 

 woods of the Potomac, to which reference should be made. 

 Every following year some papers describing fossil woods have 

 appeared, but the next contribution of real importance is 

 perhaps that of Conwentz (1892), who describes the remains of 

 Pinus from amber very elaborately and with good illustrations. 



In 1905 Gothan published his work on fossil woods, which 

 has doubtless done much to stimulate research in wood-structures 



