82 SEAWEEDS 
thick, pitted walls, which appears to discharge the 
special function ofa supporting tissue. In Macrocystis 
it is the inner portion of this layer that develops into 
sieve-tubes, of which the sieve-plates become endued 
with a callus-like coating. By the periodical addition 
of such tissues the old stalks of Zaméinariacee 
acquire the appearance of the stems of woody 
Dicotyledons. Occupying the central portion of 
the stalk there is a dense plexus of branching 
anastomosing filaments, which probably acts as a 
conducting tissue. It is gradually reduced in bulk 
upwards, and enters the midrib of the leaf, when 
present, as a thin strand. In several Laminariacee 
there are formed definite mucus-passages, mostly in 
the form of branching anastomosing tubes, and these 
are frequently bordered by other cells differentiated 
from those of the adjoining tissues. 
The Reproductive Organs are zoospores produced in 
elongate, sac-like, unilocular sporangia of the same 
shape as those of the Splachnidiacew. They occur,as 
has been mentioned, in more or less localised sori, 
mixed with paraphyses. On the one hand, in Chorda, 
the sporangia occur over the whole surface; on the 
other, in Alaria and its allies, they are restricted to 
special sporophylls, as described above. In Postelsia 
and Macrocystis the sporangia are borne in the longi- 
tudinal furrows in the fronds. The paraphyses, unlike 
those of Splachnidiwm, are non-septate, and in all 
cases, except Seccorhiza and Chorda, possess a peculiar 
hyaline appendage at the tip. Reinke has placed 
Chorda outside the Laminariacee on account of its 
cylindrical thallus and the distribution of its spor- 
