Laminaria saccharina. 
317 
plates, and its formation throughout the length of all classes of sieve-tubes 
I have been unable to confirm Oliver’s conclusion that here also callus is 
due to mucilaginous degeneration of the cell-wall. Oliver may have been 
led to this view by working on herbarium material, but in the cpurse of the 
present research all the evidence obtained from Macrocystis and Laminaria 
has pointed to the conclusion that the callus is directly laid down by the 
protoplasm, and thus supports the views of Gardiner 1 and Hill 2 . It is only 
necessary here to refer to the description on p. 302, and to Figs. 23, 27, 
and 17-20, PI. XIX, Figs. 21 and 40-42, PI. XX, in order to show how 
difficult it would be to harmonize such appearances with the view that 
the callus is produced from the already formed cell-wall. No cases were 
observed in which the callus was seen to fade gradually into the cell-wall, 
as described by Oliver 3 , and as also shown by Hill 4 in an abnormal case in 
a sieve-tube in Finns. 
As in Finns , the unaltered portions of the old cross wall are still 
visible in the oldest sieve-plate and are seen in longitudinal sections as 
beads, of a glistening appearance, which do not stain with callus-stains. 
The callus-cushion attains a very large size (Fig. 19, PI. XIX), but the 
dimensions of the original cell-wall diminish very slightly, if at all, and it is 
impossible to believe that the minute layer of cellulose which could be thus 
accounted for can be responsible for the production of such a large mass of 
callus. 5 
VI. Comparison of the Anatomy and Histology of 
Macrocystis pyrifera and Laminaria saccharina. 
In the young plants of both Laminaria and Macrocystis the original 
central cells become stretched and give rise to elongated elements which 
have been described as ‘ trumpet hyphae ’ by other authors. 6 As suggested 
above, it seems better to term them ‘primary pith filaments’. They 
probably function for conduction in the young plants ; in Macrocystis they 
are very early obliterated by callus, but in Laminaria this does not take 
place till a very much later stage. 
The stretching process begun in the central cells of the thallus is later 
continued in the primary cortex. In Laminaria the cells of this tissue 
elongate and are transformed, without further cross division, into secondary 
sieve-tubes ; in Macrocystis each cell divides transversely several times 
during the process of elongation, and here also all the primary cortex finally 
gives rise to secondary sieve-tubes. Thus in both plants the secondary 
1 Gardiner, Observations on constitution of callus, Proc. Camb. Phil. Soc., v, 1885, p. 230, and 
cf. Gardiner and Ito, Mucilage-secreting hairs in Blechnum and Osmunda , Annals of Botany, i, 
pp. 33 et seq., and p. 39 a, Figs. 34, 36, 41, 43. 
3 Hill, 1. c., 1901, pp. 597-600, and cf. Figs. 22, 26, 27, PI. XXXIII. 
3 Oliver, 1. c., 1887, Figs. 2, 10, n, PI. I and II. 
4 Hill, 1. c., 1901, p. 598, Fig. 25, PI. XXXIII. 5 Ibid., p. 599. 
6 Pp. 298 and 307. 
