the Sieve-tubes in Laminar ieae . 109 
work by — I am inclined to think — an alteration of the wall 
itself. In the figure the pores, though considerably narrowed, 
are not as yet obliterated. For the sake of clearness I have 
omitted to represent the protoplasm, continuous from one 
member to another through the pores. Fig. 18 is an in- 
teresting case of unequal thickening of the callus-plate on one 
side (from M. Orbignyana ) ; such are quite rare. 
I have never found any connection between the trumpet- 
hyphae and true sieve-tubes either in Macrocystis or in Nereo- 
cystis, though they run often very close together. 
Before concluding this paper it will be well to describe in 
detail the micro-chemical reactions given by the callus. We 
can then see how we are justified in considering it identical 
with the callus of phanerogamic sieve-tubes. 
Nature of the Callus. — It has been already stated that 
the micro-chemical reactions shown by the callus occur- 
ring both in the trumpet-hyphae and in the sieve-tubes 
proper of Macrocystis and Nereocystis agree almost abso- 
lutely with those characterising the callus of the phanero- 
gamic sieve-tube. These it will be remembered are such 
that callus cannot be confused with any ordinary mucilage. 
The following are the results given by the callus in the two 
genera : — 
(1) Rus sow's callus-reagent . — This consists of a mixture of 
equal parts of chlorzinc-iodine and iodine in potassic iodide ; 
it was used with great success by Russow and by Wilhelm, 
and is considered to be the most delicate callus-test. With 
it callus is stained a deep brown. This was the case with 
the callus of Macrocystis and Nereocystis. The unaltered 
cell-walls stain yellow. When mounted in this reagent the 
partially obliterated perforations show up very well, and 
in early stages of callus -development the thin layer of 
callus could be easily distinguished from the cellulose sieve- 
plate. 
(2) Cor allin-soda, — This reagent is due to Szyszylowicz 1 . 
1 Szyszylowicz, Osobne. vobicie z. Rospan Akad. Umiej. w. Krakowiz, x. 1882. 
