the Sieve-tubes in Laminarieae. 
99 
30 cm. long. At its upper end the stem suddenly swells 
into a round bladder-nut. On this pneumatocyst are borne, 
as a rule, five petioles, each of which gives rise to a tuft of 
leaves, the leaves at this stage not exceeding 60 cm. in length. 
With advancing age the general aspect alters very much, the 
stem becomes immensely long, without any proportional 
increase in thickness, its diameter not being more than six to 
eight millimetres. The vesicle changes into a retort-shaped 
cylinder 2 metres long and 15 cm. in diameter, its lower end 
imperceptibly passing into the stem. The leaves also divide 
actively and attain an immense length. There may be as 
many as fifty leaves each 8 metres long. So far as Mertens 
could ascertain, Nereocystis is an annual. In autumn it is cast 
up by prevailing storms on the beach and here decays, so that 
next spring no traces are left. 
The stem may reach a length of 80 metres, and is used 
as a fishing-line by the Aleutians. Nothing is known of its 
reproduction. 
Anatomy of Stein . — Examining the transverse section of a 
young stem, which has as yet only reached a length of 30 cm., 
and a diameter not exceeding 5 mm., the tissues are differen- 
tiated into (1) a central hyphal-strand or medulla of circular 
or slightly oval form, its longest diameter one-fourth of that of 
the whole stem, (2) a. broad cortex, and (3) a limiting layer or 
epidermis. 
The medulla consists of a meshwork of hyphae, embedded 
in mucilage. Many of these hyphae may be seen running 
horizontally and taking their origin from the inmost cortical 
cells. Running longitudinally, and frequently branching, the 
‘ trumpet-hyphae ’ are found in considerable numbers. Already 
at this period many of them are becoming callous. At the 
periphery of the medulla is the zone of true sieve-tubes making 
its appearance. These are arranged in radial rows about three 
deep, but in older stems up to six or eight. Between these 
sieve-tubes, which have a comparatively wide lumen as in 
Macrocystis, strands of hyphal tissue run. At present they 
are quite without callus. The medulla passes gradually over 
