56 BOTANICAL GAZETTE [JANUARY 
have been colored yellow or brown; the whole bundle is colored 
a yellowish brown or black. Nearly every bundle for some distance 
above the killed portion presents this appearance, while only occa- 
sionally one or more are seen colored below the region. The phloem 
is the part most deeply colored, being very dark brown and often 
almost black. A disorganization of the substances in the sieve 
tubes has apparently been caused by the heat; these substances can _ 
be seen to have penetrated the cell walls and to have passed out 
into the other elements of the bundle and also into the parenchyma 
cells. Under the microscope the substance which plugs the vessels 
appears like the mucilage or gum-resin so often present in liverwort 
gametophytes. It is yellowish in color and gelatinous in consist- 
ency; on testing with alkanin it is found to give the characteristic — 
red reaction of resin. The xylem vessels also appear to be plugged — 
with this same resinous substance. The smaller pitted vessels a5 _ 
well as the larger ones show this plugging, also the spiral and annular 
vessels, and even the large air spaces. It varies in degree; some- — 
times all of the xylem will be stopped, and then again only one 
or more vessels. The walls of the vessels also show the presence 
of this substance, and it is even present for some distance out in 
the surrounding parenchyma cells. When unstained with alkanin 
it is yellow or brown, depending upon the amount present; the — 
deepest color is in the phloem, where it is nearly black, which goes to 
show that the substance originates here and diffuses out into the 
surrounding tissues. That it is not present in the killed portion 
itself is probably due to the fact that the heat vaporizes or expands 
the liquids and forces them upward and downward in the sieve — 
tubes, and that the lateral diffusion into the surrounding tissues 
occurs later. This substance becomes visible very soon after steamy 
ing the stem. 
From the above observations there can be no doubt that in this _ 
plant substances due to steaming are exuded into the water passage : 
which may possibly obstruct the upward flow of water and prevent | 
an adequate amount reaching the transpiring leaves above. These — 
facts alone may possibly be sufficient to account for the diminished : 
transpiration and diminished water content of the leaves indicated — 
in tables V and VI. Whether this clogging accounts wholly for 
