378 Boodle . — On some points in the 
to the structure of the root will be taken first. Then 
indications of secondary thickening in the root and stem of 
Ophioglossum will be described, and the concluding part will 
contain a summary of previous work on the points dealt with. 
i. Structure of the Root. 
The roots of Ophioglossum vulgatum run horizontally in the 
soil, so one may conveniently speak of the upper and lower 
sides of the root. The xylem-mass of the mature stele is 
roughly semicircular in outline 1 and occupies the lower 
half of the stele, the upper half being occupied by the phloem 
and conjunctive parenchyma (see Fig. 9). 
The differentiation of the xylem from the procambial strand, 
as seen near the apex of the root, begins with the formation 
of a single tracheide on the lower side. It is shown at px 
in Fig. 1, where it is placed rather to one side of the middle 
point of the arc of cells, which afterwards forms the lower 
limit of the primary xylem. This is the usual position of the 
first protoxylem-element, though it is sometimes found at the 
middle point of this arc. 
Comparison with sections further from the apex showed 
that in this root the second tracheide is developed in the 
position of the cell marked a in Fig. 1, and that the third 
tracheide is formed between the first and second but nearer 
to a. The second tracheide is sometimes placed as given for 
this root, but it may occur immediately adjoining the first, or 
in other positions. 
To generalize from several series of sections cut through 
root-tips, one may say that the differentiation of the primary 
xylem always begins with the formation of a single tracheide 
at the periphery of the procambial strand, and then other 
tracheides are formed, in the same peripheral arc, but in no 
definite order, until it is complete, when the further develop- 
1 The shape of the xylem mass is variable. 
