308 Sykes. — Anato 7 ny and Histology of Macrocystis pyrifera and 
perforations are so extremely small that it seems impossible that groups of 
threads can have preceded them, and perhaps the single threads originally 
present never divide in such sieve-plates to form groups. In some young 
lateral plates, however, groups of threads have been distinctly seen. 
Besides these lateral sieve-plates, small isolated pads of callus are often 
found on the lateral walls of the sieve-tubes. A highly magnified example 
of one of these is shown in Fig. 22 c, PI. XIX, and no threads or slime-strings 
can be seen in it. These lateral pads often appear to be sunk right inside 
the wall, as shown in Fig. 22 a, PL XIX, but sometimes each pad appears to 
surround a hole, the edges of this hole being continuous with the inner wall 
of the sieve-tube (Fig. 22 d , PI. XIX). In the lateral walls of youngsieve-tubes 
a few cases have been found in which a small callus-ring was present and a 
collection of deeply staining protoplasm was noticed lying adjacent to the 
ring. I think it probable that these pads were originally developed round 
pits, the remains of the pit being still visible in such cases as Fig. 22 d, PL XIX. 
As stated above 1 , the pits in the longitudinal walls of the primary inner 
cortical cells were very rare and widely separated, and after the stretching 
of these cells to form sieve-tubes they naturally became still further apart, 
and, while some of them gave rise to cross connexions, others might perhaps 
be ruptured by the rapid growth of the surrounding walls, and callus-pads 
might then be formed round the old pit entrance. The first evidence of such 
a pad appears as a ring surrounding a pit; this ring then spreads over 
the pit-closing membrane, and finally a small mass of callus is developed 
which nearly or completely fills the pit. 
iv. Nature of the callus in Macrocystis . 
The callus in the primary pith filaments, secondary sieve-tubes, and 
hyphae of Macrocystis gives all the reactions characteristic of the ordinary 
callus found in the sieve-tubes of Phanerogams. In addition to those 
already enumerated by Oliver 2 , London blue, like Water blue, gives it a fine 
blue colour, Congo red stains it a bright pink, and Thionin gives it a purplish 
tinge. The fact that Thionin also stains mucilage a similar, though deeper, 
purple is of interest in connexion with the conception of callus as a 
hydrated or mucilaginous form of cellulose. The frothy appearance of 
the callus in Macrocystis , an appearance which is increased in swollen 
material, is very striking, and an attempt to figure this is seen in a lateral 
pad shown in Fig. 22 c , PL XIX. 
IV. Laminaria saccharina. 
This species of Laminaria , though simpler than Macrocystis in many 
respects, is essentially similar as regards its general anatomy, while the 
histology of the two plants differs only in degree of development and com- 
plexity. I therefore propose to give but a short account of the anatomy of 
1 PP- 2 99~3 oa 9 Oliver, 1. c., 1887. 
