248 Campbell.— Contributions to the 
At a somewhat later stage (Fig. 44) the primary tissue- 
systems are plainly seen, and are arranged as follows. 
Occupying the axis of the root is a blunt cylinder whose 
terminal cells ( x x ) are somewhat larger. This is the ple- 
rome-cylinder. At the apex the cells are nearly iso-diametric, 
but lower down become longer and narrower by the formation 
of longitudinal walls and increase in length due to growth. 
Covering the apex of the plerome is a single layer of cells, 
which is the initial for all the other tissues, and this arrange- 
ment, when established, continues as long as the root grows. 
From this layer additions are made to the root-cap at regular 
intervals, and the layers of cells so cut off do not undergo any 
further division by periclinals, but remain one cell in thick- 
ness, so that a regular stratification of the root-cap is always 
noticeable. At the apex of the root there is no distinction 
between dermatogen and periblem, but these first become 
separated back of the apex. 
The primary xylem consists of very delicate spiral tracheids 
which are formed at the base of the root at the same time that 
the first ones appear in the leaf. 
The root grows rapidly in length and bends downward so 
as to attach the young plant to the ground. It increases but 
little in diameter, and has fewer and much smaller air-spaces 
than are found in the leaf. 
If we compare the structure of the roots with that of other 
plants, it is found to correspond most nearly with that of 
certain monocotyledons. Bruchmann 1 gives for the roots of 
the older plant a structure corresponding to De Bary’s 2 third 
type of angiospermous roots, which differs from the account 
given here in that there is a special initial layer for dermato- 
gen and calyptrogen. I have not examined the older roots, 
and so cannot state whether this is true in the species under 
consideration or not. 
The Foot. The foot, which at first projects but little, 
enlarges as the embryo grows, by the rapid growth of its 
1 Jenaische Zeitschr. fur Naturw. VIII. p. 522. 
2 De Bary, Comparative Anatomy. 
