ii2 Oliver. — On the Obliteration of 
corresponding great increase in the diameter of the stem. 
No other sea- weed approaches them in this respect, and it is 
significant to note that in none other than these two are true 
sieve-tubes — in addition to mere trumpet-hyphae — known to 
exist. 
I must reserve a full discussion of the question of the 
origin of callus in sieve-tubes generally until such time as I 
may have had opportunity of examining its development in 
Macrocystis etc. in fresh material. Here I can only repeat 
that I am strongly of opinion that, in the case of the trumpet- 
hyphae, it is formed by an alteration of the cell-wall itself. 
This view will be fortified by an examination of the figures 
attached to this paper — especially Figs. 7, 8, 9 and 10. In the 
true sieve-tubes it is at present impossible to give a decision, 
though one might infer that, since in the trumpet-hyphae it is 
formed from the wall it has a similar origin in the sieve-tubes 
proper. All I can safely say is that, so far as I have gone, my 
results on the origin of callus in Laminarieae tend to confirm 
the opinion of Wilhelm and Janczewski, referred to on 
page 96, rather than the opposing view of Russow, Fischer, 
and Gardiner. 
It is not, I think, without some instructiveness to draw a 
certain comparison between these two sea-weeds — Macrocystis 
and Nereocystis — and climbing or twining Phanerogams. Like 
the climbers, both differ from their allies in that the stems 
have an extremely small diameter when considered in relation 
to the length of the whole plant. In neither does the stem 
develope mechanical tissues necessary to support it. The case 
is very different in, say, Lessonia — an allied Laminaria — which 
stands erect like a submerged tree, with stem almost as thick 
as a man, with long-continued secondary growth in thickness. 
Just as the climber is supported by its special organs of 
climbing, so are these two supported by their floats. In 
Macrocystis it is the leaf-petioles which are modified in this 
way, in Nereocystis the whole upper portion of the stem is 
dilated and forms one gigantic pneumatocyst. 
In climbers there is, as a rule, a tremendous development of 
