Laminaria sac char in a. 
323 
Fig. 9 is a diagrammatic representation of the changes undergone by a single group of threads. 
(Letters as in Fig. 8.) 
Fig. 10 is a lateral sieve-plate seen partly in section, partly in surface view. Single callus-rods 
are seen traversing the sieve-plate, x 750. 
Fig. ir. Very young sieve-tube in longitudinal section showing single threads traversing the 
sieve-plate, x 600. 
Fig. 12. As Fig. 11, but the central portion of each thread more deeply stained, x 600. 
Fig. 13. Young sieve-plate, showing single threads in some places, and groups of twos and threes 
in others, in longitudinal section, x 750. 
Fig. 14. First step in callus formation, in a young sieve-plate in which the threads are arranged 
throughout in groups. Callus is found in three places, but in each case has only been formed on one 
side of the middle lamella, x 750. At ( a ) two pairs of deeply staining threads are seen, in one of 
which the change has not yet reached across the middle lamella. 
Fig. 15. An unusual case of a sieve-plate in which development has proceeded further on the 
centre than on the periphery. On the left of the section are seen groups of two threads, and in one 
of these groups a median node is visible on each thread. Next to this group comes one made up of 
three threads each of which has its own callus-rod ; next to this, one in which two slime-strings are 
present, the formation of which is not yet complete across the middle lamella ; in the centre is a case 
in which the single slime-string is nearly complete, but across the middle lamella two separate darkly 
staining threads still stretch. On the right of the plate the outermost pit but one shows a case in 
which callus and slime-string formation is complete on the upper side, while threads without callus 
are still present on the under side, x 1,000. 
Fig. 16. In this sieve-plate fully formed slime-strings, each enclosed in a callus-tube, are seen 
on the periphery in longitudinal section. In between the periphery and the centre all stages in their 
formation are present. At ( a ) is seen a very common phenomenon : the half-formed slime-strings on 
one side dragged out of the pits during the processes of staining, &c. In many places in this sieve- 
plate the callus has spread over the areas between the perforations. The protoplasm of the sieve-tube 
is massed in the centre, x 750, 
Fig. 17. First stage in the obliteration of a sieve-tube: an accumulation of callus has occurred 
at the centre of the plate, x 750. 
Fig. 18. An old sieve-tube in which is seen the commencement of callus deposition on the side- 
walls. This preparation was stained to show the much swollen inner layer of the wall, ml 
= middle lamella ; x = lateral plate, finely perforated. (Stained Thionin and London blue.) 
x about 500. 
Fig. 19 represents one of the oldest sieve-tubes in longitudinal section. Obliteration of the 
sieve-plate is here complete and callus has also been laid down in considerable quantity on the 
lateral walls. At («) a portion of protoplasm is seen entirely enclosed by callus, clearly showing 
that the callus cannot here be due to an alteration of the wall, x about 450. 
Fig. 20. a , If, c, d, are cross sections of sieve-tubes showing various stages in the blocking of 
their lumina with callus. x 600. 
Fig. 21. As Fig. 20, showing lumen almost obliterated, x 600. 
Fig. 22. a, b, c, d , are cross sections of sieve-tubes showing the formation of lateral pads of 
callus. The pad in 22 d is seen to surround a hole, and the edges of the hole are continuous with 
the inner edge of the sieve-tube wall. Fig. 22 c is one of these lateral pads very much enlarged, 
and an attempt is made to show the flocculent nature of the callus. 
Fig. 23 is a longitudinal section of one of the innermost sieve-tubes and shows the unequal 
deposition of the callus on the lateral walls. 
Fig. 24. A longitudinal section of the cross wall of one of the anastomosing hyphae of the 
medulla. The connecting-threads are shown as fine lines traversing the wall, x 600. 
Fig. 25. As Fig. 24, showing callus formation at a cross wall. 
Figs. 26-32 are drawn from the young apex of Macrocystis preserved in spirit. Primary pith 
filaments. 
Fig. 26. A longitudinal section of a young primary pith, filament in which no callus has yet 
been formed. The knobs on either side of the sieve-plate probably represent developing slime- 
strings. x 600. 
Fig. 27. A slightly older element in which the development of callus has begun both at the 
sieve-plate and on the lateral walls of the sieve-tube, x 600. 
