Laminaria saccharina . 
3i3 
(Fig. 4, PI. XX). On the longitudinal walls the threads are restricted to 
well-defined pits. 
In the primary cortex of the young plant no threads could be de- 
monstrated, owing to this material having been preserved in spirit. No 
definite pits can be recognized in this region, and it is to be supposed that 
the threads would not be sharply differentiated into groups as in the 
secondary cortex, but that they would be evenly distributed as in the 
young sieve-plates. 
b. Hyphae. At the point of origin of a hypha the threads are arranged 
in four or five rather indefinite groups, as seen in surface view in Fig. 12, and 
in section in Fig. 14, PI. XX. In the winter material, callus was found 
to be developed to a small extent at the origin of the hyphae, and, as 
described below in the sieve-plates, each thread produces its own callus- 
rod (Figs. 13, 15, PI. XX). 
Connecting-threads were also seen in the transverse septa of many 
of the hyphae of the medulla, and here also callus was often found in the 
winter material (Figs. 22, 24, 25, 27, PI. XX). 
iii. Development of the sieve-plates and obliteration of the sieve-tubes. 
(a) In the square-ended outer secondary sieve-tubes, derived from the outer 
layers of the primary inner cortex, the cross walls were found, in longitudinal 
section, to be traversed by numerous small groups of threads, seen in sur- 
face view to be arranged in various ways to form a larger pattern (Fig. 10, 
PI. XX). In the summer material these threads stain with Safranin, in 
exactly the same manner as do the cortical threads, but in the winter 
material a deeper stain is taken by the threads of some of the inner of 
these sieveqdates, and, when a section is placed in London blue, each 
thread is found to be enclosed in its own callus-rod. Cases of callus 
formation in these secondary sieve-tubes are shown in section in Figs. 17, 18, 
20, PI. XX. The heads of the separate callus-rods are generally fused to 
form a small patch over each group of rods, but as a rule there is no great 
accumulation of callus. Fig. 21, PI. XX, is taken from a very exceptional 
sieve-plate, situated on the innermost edge of a row of the outer secondary 
sieve-tubes, and consequently next the sieve-tubes of the medulla ; a large 
mass of callus has been deposited on one side of the plate. In summer 
material a small amount of callus was twice found in elements in a position 
similar to this last. 
On the lateral walls of the outer secondary sieve-tubes, pits traversed 
by threads are sometimes found (Fig. 18 a, PI. XX). Larger lateral plates are 
present on the longitudinal walls of the inner of these elements, and some 
of these at any rate represent the origin of hyphae, while others are found 
at the origin of the larger cross connexions (Fig. it, PI. XX). 
(b) Sieve-tubes of the medulla . There is a gradual transition from the 
Y 
