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DUNCAN S. JOHNSON 
smaller, ringed duct 7 or 8 thick, lies on each side of this (figs. 5, 6). 
On each side of the plane in which these three ducts lie is seen a 
small group of phloem elements, apparently containing only one or 
two sieve tubes each (fig. 6) . The whole thus makes up a very simple 
diarch bundle. The other elements of the stele are thin-walled cells, 
about 8 cr 10 ju in diameter and 100 to 140 jjl long. There is no evi- 
dence in the oldest roots seen of the formation of any cambium zone, 
nor even of the development of any secondary xylem or phloem in the 
bundles (figs. 5, 6). It is evident that in this creeping plant new 
water-absorbing and water-conducting tissue is added, not by the 
development of secondary tissue in roots already present but by the 
formation of entirely new roots on younger portions of the plant. 
The time of persistence of the roots was not determined. 
The internodes of the delicate, sparingly branched stem are also 
extremely simple in internal structure. The epidermis, as seen in 
transverse section is made up of a single layer of cells which are about 
15-30 fjL wide and thick and 40-100 fx long. Interspersed with these are 
occasional rather cubical oil cells, and the similarly shaped cells which 
bear, or have borne, the two-celled, knobbed hydathodes (figs. 10, 
12, 13). The latter are identical in structure with those shown in 
various stages of development in the figures of flower and fruit. The 
outer walls of the ordinary epidermal cells are but slightly cutinized, 
but this thin cuticle is peculiar in being thrown into numerous minute 
corrugations (fig. 13). The middle, cuticular layer of the outer cell 
wall, often 2 thick, is much thicker than the outer cutin layer, 
while the inner cellulose layer has only twice the thickness of the cutin. 
The outer walls of the oil cells are very similar in thickness and struc- 
ture to those of the epidermal cells, while the outer walls of the 
hydathodes are very thin and without distinguishable layers. 
The cortex of the mature internode consists, in addition to the 
epidermis, of four or five layers of thin-walled cells which vary from 
30 to 200 fjL in diameter and from 150 to 250 ju in length. Small air- 
spaces are present, especially between the cells of the outer layers. 
The endodermis consists of but 18 or 20 cells in transverse section. 
These have a diameter of from 20 to 60 m and a length of 50 to 70 ix 
(figs. 10,11). The radial walls are but slightly thickened , and the crink- 
ling is coarser than that noted in the endodermis of the root. The 
whole central cylinder of an internode, within the endodermis, is 
150 to 250 iJL across, and is often flattened in the plane of the leaf next 
