Anatomy of the Osmundaceae. 529 
presence of some tannoid substance which acts as a mordant, for they are 
coloured blue by cupric acetate and reddish-brown by bichromate of potash. 
Cells containing tannoid substances, and in consequence staining exactly 
the same as those of the endodermis, also occur in the very earliest formed 
pith, and at first sight might easily be taken asendodermal cells (PL XLIV, 
Fig. 5). They are scattered about irregularly and are also found in the medul¬ 
lary rays and in the xylem-sheath, both in the axil of the leaf-trace and for 
some distance out into the free petiole. In the later formed regions of the 
sporeling these cells are no longer present. The earliest formed cells of the 
pith are considerably elongated, almost as long as the tracheae, but above 
they soon become approximately isodiametric. The isodiametric cells, 
however, give no indication of having been derived from the septation of 
elongated cells in the apical meristem. No especially short tracheae were 
found towards the centre of the xylem, such as might have been expected 
from Thamnopteris and Zalesskya , either before or after the appearance of 
a pith. The cortical tissue of the stem is all more or less sclerotic below 
the first leaf. Above this level it consists of 4-7 layers of heavily sclerotic 
outer cortex surrounding a single layer of thin-walled inner cortex. 
Structure of the Leaf-trace. 
The size of the earlier leaf-traces varies so much from one seedling to 
another that the following must be regarded as an average account only. 
The xylem of the first leaf-traces is always very scanty and often no 
definite protoxylem could be distinguished. Probably the tracheae differ¬ 
entiated all simultaneously or else quite irregularly. Whenever a definite 
protoxylem could be determined in these small traces it was usually endarch. 
In a number of traces, however, the protoxylem was unmistakably mesarch 
(Fig. 5-8). In all, thirteen undoubtedly mesarch traces were found which 
belonged to seven different sporelings. Several other traces also showed 
a strong tendency to mesarchy. The majority of the mesarch traces 
belonged to leaves lying between the third and the sixth, two belonged to 
the second leaf and one to the eighth. Most of the mesarch traces^ were 
fairly large, containing from fourteen to twenty tracheae, but two contained 
only eight tracheae. All the mesarch traces eventually became endarch if 
followed out far enough into the free petiole (Fig. 9). The mesarch con¬ 
dition sometimes continued all the way from the stele far out into the 
stipular leaf-base, or it was only present while passing through the cortex of 
the stem, or sometimes only in the basal region of the stipule. The mesarch 
traces are more or less oval in transverse section, and it should be noted 
that the departure of such a leaf-trace does not leave a pocket in the xylem 
of the stem because it is not hollowed out adaxially at the point of its 
departure. 
