354 Barber . — Cnpressinoxylon vec tense. 
In the fossils under examination the medullary rays are 
uniform in structure, consisting of parenchyma alone ; they 
are also generally one cell thick (excepting in root (2), which 
should perhaps be separated because of this and other 
differences in the medullary rays). 
The characters mentioned thus far are of great importance 
in descriptions ; as regards the constancy of those to follow 
we are a good deal in the dark. The earlier writers, in 
describing the medullary rays, were content with mentioning 
whether they were simple or compound, and indicating the 
height they might reach in cells vertically above one another, 
e.g. 1-18, 1-6, 2-36, &c. But the uselessness of these 
numbers will be at once apparent to any one who has 
examined a number of tangential sections of Coniferous 
wood, the maximum height of the medullary rays depending 
largely on the time taken in their examination. It has 
moreover been demonstrated that in this respect medullary 
rays vary according to their distance from the pith and 
height up the stem ; and stems, branches and roots of the 
same plant may have medullary rays of different heights 1 . 
Nevertheless we do meet with cases where the height of 
the medullary rays is more or less characteristic of the wood. 
They are usually low in Thuja , higher in Cupressus than in 
Juniperus , in Abietinae they have half the number of cells 
they have in Cupressinae (except Thuja :), &c. It is there- 
fore necessary to include the height of the medullary rays in 
all diagnoses; especially in those cases where we have a know- 
ledge of the age of the part and its morphological value. 
A similar series of remarks is applicable to almost every 
one of the characters of medullary rays which different 
writers have from time to time attempted to introduce. 
Their frequency in the wood, the height and width and 
radial length of the individual cells, the thickness and 
pitting of the walls and the general shape in cross-section — - 
all these have been demonstrated to be valueless for the 
1 Essner, Ueber den diagn. Werth der Anzahl u. Hohe der Markstrahlen bei den 
Coniferen. Abh. d. natur. Gesell. zu Halle, xvi, 1886. 
