Laminaria saccharina . 
3i9 
of the sieve-plate in that plant. In Laminaria the single threads traversing 
the young plate are arranged in more or less well-defined areas. Each 
thread develops its own callus-rod and gives rise to a separate slime-string, 
and, though the heads of a group of these rods are often united, the rods 
themselves always remain distinct throughout the thickness of the plate. 
In other words, each thread produces a separate perforation in the sieve- 
plate. In Macrocystis the threads are homogeneously distributed in the 
very young sieve-plate, and are not confined to any special areas. Each 
original thread soon divides to form a group, and in the first stage of callus 
formation each thread of such a group has its own callus-rod ; but from 
each group of threads with their separate callus-rods is finally developed a 
single slime-string enclosed in one callus-rod. In other words, each thread 
of a young sieve-plate eventually gives rise to a separate perforation in the 
sieve-plate, but, in between the original and the final state, a stage is found 
in which a group of threads is present, derived by the division of the single 
thread, and giving rise by fusion to a single slime-string. 
In Macrocystis and Laminaria the old sieve-tubes are obliterated in 
very much the same way by the deposition of callus. In the former plant 
the callus begins to accumulate at the centre of the sieve-plate in a single 
mass and spreads thence to the periphery ; in the latter, callus begins 
to be formed over each area of threads in numerous places on the sieve- 
plate, and then spreads in every direction over its surface. 
Rarely in the summer, more often in the winter, callus is found in 
Laminaria which has the same properties as that of Macrocystis , but callus 
in a lower state of hydration is more often present in Laminaria. 
The structure of the general cell-walls and of the pit-closing membranes 
is essentially the same in the two plants. 
VII. Other Species examined. 
I have examined various other species of the Laminariaceae, by way 
of supplementing the results obtained during the above research, and I 
propose to refer to them shortly here. 
Sacchoriza {Laminaria) bulbosa , De la Pyl., has no distinct medulla 
and no sieve-tube-like elements. Some elongated cells, with living con- 
tents and very thick walls, are found in the central part of the stem ; these 
do not appear to anastomose with each other or with other elements. 
Laminaria digitata , Lamour., of which the material examined was 
preserved in summer, is very similar in structure to Laminaria saccharina. 
Its stem is differentiated into cortex and medulla, and numerous sieve-tubes 
are found in the medulla. The inner cortical layers, which resemble in 
position and appearance the outer secondary sieve-tubes of Z. saccharina , 
give rise to hyphae which grow into the medulla. In the innermost cells 
of this tissue exceptional cases of callus formation were found ; in two 
