Laminaria saccharina . 
3 2 1 
primary cortex and, both in position and mode of development, are 
undoubtedly homologous with those well-known elements. The greater 
development of the conducting organs in Macrocystis and Nereocystis is 
probably due to the habit of those plants. 
III. The histology of the sieve-plates, in the primary pith filaments 
and secondary sieve-tubes, is essentially the same. Threads are found 
traversing the young sieve-plate, and each gives rise in the older plates, 
apparently by means of ferment action, to a slime-string enclosed in a 
rod of callus. In Macrocystis each original thread first divides to form a 
group, and each thread of a group forms its own callus-rod, but finally, 
by fusion of these, only one slime-string is produced from each group. 
The older sieve-plates are obliterated by the deposition of callus in large 
masses over their surface, and callus is also formed throughout the length 
of the old sieve-tubes. 
IV. Callus is to be looked upon as a hydrated form of cellulose, and is 
found in Laminaria saccharina and L.digitata in various states of hydration. 
It appears to be produced in the young sieve-plates by the action of a fer- 
ment on the already formed cell-wall, but is afterwards accumulated by 
deposition from the protoplasm, both on the surface of the sieve-plate and 
on the lateral walls of the tube. 
V. It is interesting to note how fully the histology of the sieve-tubes 
agrees with that of the sieve-tubes of Phanerogams. It is observed that, 
at the advent of the callus, a simultaneous increase of staining capacity 
becomes noticeable in the threads, and, as in Pinas 1 * , it is suggested 
that the development of the sieve-plate is a function of ferment action. 
There is one point of contrast between the method of obliteration of the 
sieve-tubes in the Laminariaceae and Pinas . In Pinas ' 1 the heads of 
the slime-strings were found to be still visible on the free edge of 
the callus-cushions, and the path of the slime-strings could be traced 
throughout the callus-mass ; but in Macrocystis and Laminar ia, on the 
other hand, the callus is laid down by the protoplasm of the sieve- 
tube, over the heads of the slime-strings, so that they are buried by the 
overlying callus, and no perforations can be traced through the pad 3 . 
VI. It has been shown that in young stages of L. saccharina the 
cells of the hyphae become secondarily attached to those of the primary 
cortex, and that this phenomenon also probably occurs in Macrocystis. 
VII. Protoplasmic connecting -threads have been demonstrated 
throughout the tissues of Macrocystis pyrifera and Laminaria saccharina , but 
it is impossible to be certain of their formation in the case of secondary 
attachments. Their demonstration in cells not genetically connected would 
1 Hill, 1 . c., 1901. 2 Ibid., 1901, II, Fig. 14, PI. XXXII. 
3 Fig. 19, PI. XIX, Fig. 39, PI. XX. 
