CYCADOFILICES FROM THE CALCIFEROUS SANDSTONE SERIES. 817 
sometimes they occur between every two rows (PI. I. fig. 8, m.r.). In one specimen, 
after the development of some fourteen rows of tracheides, there is a sudden change to 
much smaller elements (PI. I. figs. 8 and 9, w.r.). Just such a change, indeed, as is seen 
between the spring and autumn wood in living trees. About three rows of small 
elements are formed and then the larger tracheides are again developed. The change 
occurs simultaneously all round the stem and not irregularly, as has been occasionally 
observed in some fossil stems. It is not suggested, however, that this proves seasonal 
alternation at that early time in the earth’s history, though probably some external 
influence had caused a temporary check to the growth. When, however, the conditions 
became favourable again, large elements were developed as before. 
Immediately beyond the xylem cylinder there occur, in certain places, patches of 
phloem in a fair state of preservation. The elements comprising this tissue have thin 
walls and are considerably elongated. No sieve plates were observed on their walls, 
but the light brown contents of the cells may have obscured them. Secretory sacs and 
ducts are sometimes seen in this region. 
Passing outwards from the phloem the cortical zone is reached, and it can easily be 
separated into two layers—the inner and outer cortex. No hard and fast line, however, 
can be drawn between the pericycle and the inner cortex. In transverse section the 
latter tissue may be seen in PI. II. fig. 10, 2.¢., and there the delicate cell walls of the 
parenchyma may easily be observed. Numerous secretory cells (s.c.) and sacs (s.s.) 
are also clearly visible. In longitudinal tangential section a great number of these 
elongated sacs (s.s.) are shown, while the small secretory cells are also very distinct 
(Pl. Il. fig. 11, s.s. and s.c. respectively). The secretory elements with their dark 
contents are so numerous that they impart a peculiar appearance to the whole inner 
cortex. One would almost imagine that the whole tissue was of a mucilaginous nature. 
Towards the periphery of this region the cells appear tangentially elongated (Pl. II. 
fig. 10, c.p.), but that is due to the outer cortex being crushed down upon the inner 
zone. As a result there is a well-defined junction between inner and outer cortex, but 
the junction is less marked where there has been little crushing (PI. II. fig. 15, a.). 
The great number of secretory elements in the inner cortex at once distinguishes it 
from the innermost layers of the outer cortex. 
The outer cortical region constitutes one of the most distinctive features of the 
genus, and in both specimens it is very well preserved. Three zones may be noticed. 
The innermost consists of a thick-walled parenchyma and is free from secretory elements. 
The second (hypodermal) zone consists of a parenchymatous ground tissue in which 
anastomosing groups of sclerotic fibres are set. These fibres are closely associated with 
secretory ducts. The ground tissue of this cortical layer is peculiar. In a transverse 
section of the stem the parenchyma is seen to contain elements which are elongated 
radially with respect to the nearest sclerotic group (Pl. II. fig. 15, eo.c.). In any 
other direction they resemble the rest of the cortical cells in size. The elongation takes 
place towards the sclerotic groups as centres (Pl. IL. fig. 15). In some cases the 
