Laminaria saccharina . 
3i5 
oldest sieve-tubes, and also in a few of the hyphae, callus is formed through- 
out the length of the cell, and here, as in Macrocystis , it seems highly 
probable from its mode of formation that it is laid down directly by the 
protoplasm (Figs. 40, 41, PL XX). 
All these stages, just described under the head of sieve-tubes of the 
medulla, were examined in winter material, but some obliterated sieve-tubes 
were found in summer material, and, after much swelling with various re- 
agents, callus was also demonstrated in a few of the younger examples. 
iv. Nature of the callus. 
The callus of Laminaria saccharina does not appear to be all of 
the same kind. That found in the outer secondary sieve-tubes, in most 
of the hyphae, and in the young inner secondary sieveTubes is only visible 
when stained with London blue, and it stains with that reagent in unswollen 
winter material, but gives no reaction with Water blue either in swollen or 
unswollen material. 
It was stated above that in the summer little or no callus was to be 
seen. This statement applies to fresh material, but it was found that, after 
being left for a considerable time in some swelling agent, such as Potassium 
iodide and Iodine, a substance which stained with London blue was 
to be seen in several of the older sieve-tubes and plates, as well as in a 
few younger stages. In summer material rare examples of callus were found 
in the innermost sieve-tubes, which stained with Water blue or Russow’s 
callus reagent, but in the winter material the callus present in all the older 
tubes, as well as that in such unusual cases as that figured in Fig. 21, 
PI. XX, stains with these reagents. 
It seems probable that callus in various states of hydration is present 
in Laminaria saccharina , for : — 
(a) Most of that found in summer differs but little from the substance 
of the ordinary cell-wall, and only gives a characteristic callus reaction after 
further hydration with swelling reagents. 
(h) In winter and in a few cases in summer the callus is sufficiently 
hydrated in the fresh material to stain with London blue. It becomes 
much more definite after swelling for some time, but it was never found to 
stain with Water blue. 
(c) Finally, the most hydrated form of callus is similar to that 
found in Macrocystis , and stains with Water blue as well as with London 
blue. It was only found in the sieve-tubes of the medulla and in the 
exceptional case mentioned above \ and shown in Fig. 21, PI. XX. This 
fully hydrated callus is hardly ever formed in summer. 
1 P- 31 1- 
Y 2 
