Knox: STEM oF IBERVILLEA SONORAE S37 
ized so as to make a distinct line. across the outer surface of the 
leptome (as in FIGURE 11) with the appearance of a normal cam- 
bium. The line does not always show distinctly and is more apt 
to occur in large bundles. 
These conclusions concerning an inner cambium do not agree 
with those of Bertrand (9), Lotar (12), and Schenck (26), who 
found hadrome formed by the inner cambium, nor with those of 
Fischer (17), who remarks the absence of an active inner cam- 
bium, saying that for a while the still procambial cells have the 
appearance of one. It is rather the state of things noted by 
Vesque (5), who claims as a false cambium one which produces 
only leptome; and afterward found by Scott and Brebner (25) in 
Thladiantha, The latter authors state that when a cambium is 
present it produces leptome only. 
Around the oldest tangential borders of the leptome-regions 
are found the horny walls of the disorganized (obliterated) sieve- 
elements as well as masses of cells filled with yellow-brown gum 
whose localization is illustrated in FIGURE 9. The breaks in the 
stereome-ring are filled up with parenchyma, and dilatation- 
changes also occur in the medullary rays and the pericycle by 
which they keep pace with the increase in the size of the sfem. 
It occasionally happens, as Hérail (19) found to be the case in 
Ecballium, that the tangential divisions of the parenchyma may be 
So localized between two bundles as to give the appearance of an 
interfascicular cambium. Potter (22) found an interesting inter- 
fascicular cambium in Zh/adiantha which connected bundles of 
the inner and outer circles, showing as de Bary had said that the 
two concentric rows function as a single ring. New medullary 
rays are not formed yearly. In large old stems two or perhaps 
three may occur (FIGURE 13), but this is in very old plants, and 
they develop only at long intervals. The periderm is superficial 
in origin. It arises from the layer of the collenchyma just pe 
neath the epidermis. It soon becomes spotted or streaked with 
deposits..of calcium carbonate in. the radial-and tangential walls 
of the phellem. The epidermis breaks away over the encrusted 
areas, and the gray color which appears in the stem ts due to the 
€xposure of such groups of cells. When the calcium carbonate 
is dissolved out with hydrochloric acid, the walls of the cells give 
the lignin reaction with phloroglucin. 
