g6 Oliver . — On the Obliteration of 
realise that such structures as sieve-tubes really occur at all 
so low in the plant scale. 
All are agreed that amongst the higher plants the sieve- 
plate becomes at an early period in its history callous , in other 
words, even before the development of the perforations, a 
substance, possessing most characteristic chemical reactions, 
and known as callus , is found forming a thin investment of the 
future sieve-plate ; and that after the perforations are formed, 
the callus being added to, becomes thickened, eventually 
entirely blocking up the pores and causing the obliteration 
of the sieve-tubes. The point in dispute is as to the origin of 
the callus, Wilhelm and Janczewski holding that it is formed 
by an alteration of the cellulose of the sieve-plate itself; 
Russow, Strasburger, Fischer and Gardiner that it arises 
rather from the contents of the sieve-tube. 
In an investigation into the anatomy of the brown sea- 
weeds, more especially of the Laminarieae , I was struck with 
the very general presence in certain genera of a substance 
entirely resembling callus in its reactions, and in this paper I 
hope by an account of its mode of origin, so far as I have 
been able to follow it, to throw some light on this vexed 
question. Before entering on my own results it will be useful 
to briefly note what has already been done within the group 
of the brown sea-weeds in the same direction. 
If a transverse section of any Laminaria (e.g. L. digitata) 
be examined a considerable differentiation of tissues will be 
met with. The closely-fitting and usually radially-elongated 
epidermal cells are succeeded by several layers of small 
parenchyma-cells, which, as well as the epidermal cells, are 
richly filled with brown pigment-bodies (phaeoplasts) ; within 
this comes a very wide zone of highly-pitted parenchyma- 
cells, extending almost to the centre. These are formed in 
a number of concentric layers, and arise by a secondary 
growth resembling very much the growth of the secondary 
wood in a dicotyledonous stem 1 . It is the central strand of 
1 Cf. Le Jolis, Examen des especes confondues sous le nom de Laminaria 
digitata , in Nova Acta Acad. Leop. Carol. 1855. 
