Laminaria saccharina. 
325 
Fig. 22. Surface view of a cross wall of an anastomosing hypha from the medulla, x 600. 
Fig. 23, as Fig. 22 displaced in longitudinal section. 
Figs. 24, 25, 26, 27. Longitudinal sections of hyphae in the medulla, in the cross walls of 
most of which callus formation has occurred, x 600. 
Fig. 28. Surface view of a young inner secondary sieve-plate from the periphery of the medulla. 
Hyphae are seen cut during their course in the walls, and a small sieve-plate is also seen in surface 
view in the left-hand comer, and probably represents the cross wall of a large hypha. The threads 
seen in end view in the large sieve-plate are confined to certain areas in the wall and are arranged in 
a star-like pattern, x 750. 
Fig. 29. Surface view of a sieve-plate from an inner secondary sieve-tube. The ends of the 
threads are arranged in smaller groups than are those in Fig. 28. x 750. 
Fig. 30. Surface view of a slightly older sieve-plate from the medulla. A few of the threads 
stain more darkly than the rest, and are each enclosed in a ring of callus, x 600. 
Fig. 31. Surface view of a portion of a sieve-plate in which perforation is complete. Each 
thread has become transformed to a slime-string, and a faintly staining ring of callus was visible 
round each string, but is only indicated in the figure by a black line. The callus has already begun 
to accumulate in three places, x 600. 
Fig. 32 a. A slightly later stage than Fig. 31 in surface view. The callus has now spread over 
all the areas of the plate which were originally traversed by threads, but has not yet been formed in 
the other portions of the plate, x 600. 
Fig. 32 b is a very much enlarged representation of a small portion of the plate, in which the 
faintly staining callus is coloured blue, and a small area left unattacked is left white. (In Figs. 31 
and 32 the slime-strings are seen to have fallen out of most of the holes during the process of 
preparing the sections.) 
Fig. 33. Longitudinal section of a sieve-tube of the medulla, showing the sieve-plate traversed 
by groups of threads, x 600. 
Figs. 34 and 35 as Fig. 33. 
Fig. 36. Longitudinal section of ‘ three-armed sieve-plate * from the medulla. Callus has been 
formed in both terminal and lateral plates, each thread being enclosed in its own callus-rod, and the 
heads of each group of threads being fused together, x 600. 
Figs. 37 and 38. Longitudinal sections of two sieve-plates from the medulla, showing callus 
formation in each group of threads and accumulation above the groups to form small cushions. 
Fig. 38 b shows one of these cushions much enlarged, x 600. 
Fig. 39. An old sieve-tube in longitudinal section ( x 600), showing a large accumulation of 
callus on one side of a sieve-plate, x 600. 
Fig. 40. A similar case in which callus formation has also commenced down the sides of the 
sieve-tube, x 600. This figure should be compared with Figs. 23 and 31, PI. I, and Fig. 41, PI. II. 
Fig. 41. Longitudinal section of a similar sieve-tube, showing callus formation on the lateral walls 
obliterating the lumen in some places, x 600. 
Fig. 42, a , b , c, d. Transverse sections of sieve-tubes and primary pith filaments from the 
medulla, showing the lumina in various stages of being blocked by callus, x 600. 
Fig. 43. Transverse section of a sieve-tube, showing origin of lateral branch or of a pulled-out 
cross connexion. Callus has been formed as on the end-walls of the sieve-tubes, x 750. 
PLATE XXI. 
(From photographs, which are described in text.) 
Fig. 44. Cross section of young stem of Macrocystis pyrifera . 
Fig. 45. Part of cross section of old stem of Macrocystis pyrifera. The radial rows of secondary 
sieve-tubes are seen on the periphery of the medulla. 
Fig. 46. Cross section of very young plant of Laminaria saccharina. 
Fig. 47. Central portion of Fig. 3, much more highly magnified. 
Fig. 48. Cross section of young plant of Laminaria saccharina. 
Fig. 49. Cross section of old plant of iMminaria saccharina , showing radial rows of sieve- 
tubes on the periphery of the medulla, and a few large sieve-tubes nearer the centre of the medulla. 
Figs. 3, 5, and 6 are all magnified to the same extent, and illustrate the tremendous increase in 
amount of the secondary cortex. 
