104 
INTERCELLULAR RELATIONS OF PROTOPLASTS. 
Bryonia dioica is shown in Plate III., fig. 1. When, on the 
other hand, the constituent cells are fusiform, and join by 
more or less strongly overlapping ends, the walls separating 
the constituent cells, and consequently the sieve plates which 
are formed on and in these walls, are oblique in greater or less 
degree (e.g., Vitis * and the great majority of sieve tubes). In 
such cases a cross section will naturally not show the sieve 
plate, or will only show an often unrecognisable fragment of 
it, and longitudinal sections are the only ones by which they 
can be studied in surface view. 
Variously constituted as sieve plates are, they show 
one apparently constant feature, to which Hanstein was, I 
believe, the first to draw attention, but which Russow has, 
in the second of the memoirs above referred to, thoroughly 
studied, as also, though to a lesser degree, did Wilhelm and 
Janczewski. This is the presence on the plate of a substance 
called callus. The normal wall of the sieve tube is soft and 
colourless, colouring blue with iodine and sulphuric acid, or 
with clilorzinc iodine, and therefore of pure cellulose. The 
end, or sieve walls, show, however, a different structure. 
The wall is manifestly thickened, and with a substance 
which does not give the above cellulose re-actions, nor does 
it dissolve in ammonia cuproxide. On the other hand, this 
substance possesses certain marked re-actions: with a solution 
of Rosolic acid (corallin) in soda or ammonia f it takes a beau¬ 
tiful rose-red colour, a colour, however, of unfortunately but 
brief duration. This reaction shows the substance to be a 
transformed cellulose of a mucilaginous nature ; aniline blue, a 
colour which is not retained by cellulose, is fixed by this 
substance (Russow); aniline brown is also fixed; a mixture 
of clilorzinc iodine and potassium iodide iodine in varying 
proportions colours it of a deep reddish brown and shows 
also any variations present in its structure; J and the same 
end I have attained with success by first treatment for a few 
minutes with potassium iodide iodine, and then for twenty- 
four hours or so with clilorzinc iodine, and without needing 
to vary the proportions so carefully from plant to plant. Of 
all these re-actions the corallin and iodine ones are the best, 
but unfortunately both are ephemeral, though the latter will 
last with care for days in all its pristine delicacy. The 
coloration by aniline blue, though apparently permanent, is 
generally of comparatively little use, as it is accompanied by 
* In Vitis the sieve plates are occasionally horizontal. 
f See Szyszylowicz, in Bot. Centralbl., 1882. 
J Bussow, Sitz. d. Dorp. Naturf. Ges., 17 Feb., 1882. 
