1078 
Thompson . — The Anatomy and 
Pith. 
The pith of Ephedra is relatively very large, as may be seen in PL XCIV, 
Fig. 1, which is a photograph of a transverse section of an entire young 
branch. It is composed of dead, fairly thick-walled cells, many of which are 
filled with a hard brown substance. This substance is either homogeneous 
or of a foam-like structure (see Fig. 1). Often the cell-walls in certain places 
disappear, so that the dark masses become continuous, and, when the stem 
is broken, project as definite strands. This material is lacking at the node 
of the adult, and in both node and internode of the seedling. In a few 
species the pith also contains thick-walled fibres resembling those of the 
bast, which are confined to its periphery. 
In certain places, notably in proximity to the nodes, the cells of the 
pith adjacent to the primary vascular bundles become differentiated. They 
are smaller and strongly lignified, and appear as definite strands extending 
inward from the protoxylem cluster. Fig. 2 gives their appearance in 
E. trifurca. It is plain that they might readily be mistaken for centripetal 
wood. Longitudinal sections, however, reveal their true character as 
lignified pith cells. It is possible, however, as Worsdell 1 holds for similar 
cells in Welwitschia , that they may represent modified centripetal wood. 
In Ephedra there is little evidence for such a view. The conditions just 
described emphasize the necessity for exercising the greatest care in 
identifying centripetal wood. 
Another peculiar feature of the pith is the presence of a diaphragm of 
periderm-like cells a short distance above the node. In Fig. 3 the node is 
to be recognized by the constriction in the pith and the layer of thick- 
walled cells. Some distance above the node, the diaphragm may be seen 
extending completely across the pith and into the wood. The diaphragm 
is composed of vertical rows of three or more cells apparently formed 
by secondary divisions. It is sharply differentiated from the rest of the 
pith by the arrangement, size, and contents of its cells. Very frequently 
the branch breaks just above the node, and this layer always forms the line 
of separation. It serves no doubt to protect the surface exposed after the 
break. But on account of its extension into the wood it would also appear 
to be a device for cutting off young branches after the manner of an absciss 
periderm in leaf-fall. It recalls strikingly the similar structure found above 
the node in Equisetum and the Calamites. 2 In these forms, however, it is 
confined to the centre of the pith, not ordinarily reaching the periphery nor 
the surrounding wood. In the Calamites there was a diaphragm both 
above and below the node. Of course these peridermal diaphragms in 
1 Vascular Structure of the Flowers of the Gnetaceae. Annals of Botany, 1901. 
2 Williamson and Scott : Phil. Trans. Roy. Soc., 1894. 
