L aminarict sacchari na. 
309 
Laminaria saccharina , and then to pass on to its histology. The latter, 
more particularly the section dealing with the sieve-tubes, is of especial 
interest in connexion with the light which it throws on the evolution of the 
complex secondary sieve-tubes in Macrocystis , since these have hitherto 
been regarded as unparalleled among the Brown Algae. 
(a) General Anatomy , ; 
Stage i. Young plant. The intercalary growing region in a very young 
plant of Laminaria saccharina already reveals a differentiation into primary 
cortex and medulla, though secondary growth by means of a cortical meri- 
stem has hardly yet begun. A cross section of such a stage is shown in 
Fig. 46, PL XXI. 
The cells of the outermost layer of the cortex are just beginning to 
divide tangentially and radially to form the secondary cortex, but the main 
part of the cortex is primary, and can be seen in longitudinal section to be 
composed of rows of cells showing a gradual transition, from almost isodia- 
metric cells on the outside, to much more elongated ones near the medulla. 
In these last, which are rapidly being stretched longitudinally, the swelling 
of the walls is more or less obvious, causing the pits in the longitudinal walls 
to become much stretched, hence giving rise to cross connexions between 
the cells. Hyphae are found arising from the inner layers of the primary 
cortex in various ways. They may originate as small projections, either 
near the cross wall of a cell, or sometimes at other places on its longitudinal 
wall, but always on the side nearest the medulla. They grow inwards 
towards the medulla and, since even at this stage free ends are but rarely 
found, it seems certain that secondary connexions are speedily formed. 
They often grow straight across, and thus connect together two inner 
cortical cells on opposite sides of the medulla, and during their course 
they may or may not give off branches. Others again run but a short 
way into the medulla, and soon form secondary connexions with other 
hyphae. 
The medulla, a more magnified transverse section of which is shown in 
Fig. 47, PI. XXI, is composed of much elongated cells, dilated at the trans- 
verse septa and with longitudinal walls much swollen, owing to mucilaginous 
degeneration. The invading hyphae anastomose in the jelly thus formed, 
but they never form connexions with the elongated elements. There can 
be no doubt that these last are derived from the original central cells of the 
thallus, and I propose to call them ‘ primary pith filaments ’, as in Macrocystis . 
They probably function as the primary conducting-cells in the young plant 
and at this stage contain no callus. 
Stage ii. Beginning of growth in thickness. The next stage investi- 
gated was found in material taken from the base of the lamina, in a young 
plant about eight inches in length. 
