84 DESCRIPTIVE CATALOGUE 



circular in transverse section, increasing towards the centre 

 of the axis, where they measure as much as 90 p in diameter. 

 Their walls are all rather thickened, and their rounding leaves 

 small, triangular, intercellular spaces between the cells. All 

 these features can be seen in PI. Ill, fig. 2. In longitudinal 

 direction the cells are somewhat elongated, some of them 

 having a length equal to twice their diameter, but most of 

 them are shorter than that. The cross-walls are approximately 

 rectangular. 



Protoxylems appear to be endarch. The small elements com- 

 posing them can be seen in longitudinal section to have 

 extremely fine scalariform and spiral thickening. 



The secondary tracheids form regular rings of elements in 

 which the distinction between spring and autumn wood is very 

 sharply marked. The spring tracheids vary somewhat in size, 

 the largest measuring about 40 x 50-55 x 50 /u, the size of the 

 tracheids in the outer rings of wood tending to be a little larger 

 than in the first few rings. The walls are all rather thick, the 

 first-formed spring elements having a wall at least 3-4 p thick. 

 The autumn * wood elements are not so much flattened radially 

 as is generally the case in gymnospermic woods, but they have 

 excessively thickened walls (see text-fig. 17). which may be as 

 much as 10-20 p thick, with a small circular lumen only 4-6 p. 

 across. Pits on the radial walls can be seen in transverse 

 section ; in longitudinal section they are principally in a single 

 row, the circular borders being about g-i| the diameter of the 

 tracheid in which they lie. They arc generally distant from 

 each other, but may be almost adjacent. A very few elements 

 have a double row of pits. A few pits can be seen in the 

 tangential walls. 



Wood- parenchyma appears to be present in very small 

 quantities scattered through the wood. I have detected in 

 radial and particularly in tangential sections a few elements in 

 which there are true horizontal walls at right angles to the long 



* It is not now usual to employ the terra autumn wood, for, physio- 

 logically, it is late summer wood ; but for purely morphological descrip- 

 tions the old-fashioned term seems to offer a better contrast, and also leaves 

 no possible confusion, for, in some of the old papers on fossil woods, the 

 term " summer" seems to be used as " spring" is used at the present day. 



