296 Sykes. — Anatomy and Histology of Macrocystis pyrifera and 
(b) Histology. 
Owing to difficulties of technique very little histological work has been 
done on properly preserved material of the brown algae. 
i. General cell-wall. The mucilaginous nature of the general walls 
has been noted by several authors and has been contrasted with the harder 
substance which forms the pit-closing membranes and the sieve-plates. The 
middle lamella is said to be largely composed of the calcium salt of tung- 
stic acid, which is analogous to the calcium pectate of higher plants. 
Rosenthal 1 gives some account of the distribution of the pits in the 
cortex of Macrocystis , where they are confined chiefly to the end walls, but 
are also present occasionally on the lateral walls ; protoplasmic threads 
have not been figured. 
ii. Development of the sieve-plate in Macrocystis. In connexion with 
Rosenthal’s hypothesis of the formation of sieve-tubes from the cortex 
of that plant, he brings forward a remarkable theory to account for the 
origin of a sieve-plate from the cross wall of a cortical cell. The latter has 
usually a single ring of pits in its end wall, but towards the pith Rosenthal 
says that cross walls are found in which the inner edges of these pits become 
indistinct, and finally the intervening wall in the centre of the ring is partly 
dissolved. Only a thin membrane is then left across the cell, having still 
a thick edge which represents the unaltered part of the wall outside the 
ring of pits. The thin membrane becomes the sieve-plate, and is finally 
found to be perforated by numerous holes, which are larger in the middle 
and smaller towards the periphery ; the origin of these holes appears to be 
somewhat obscure. Rosenthal found but few lateral plates on the walls of 
the sieve-tubes, and notes that the lateral pits of the cortical cells would 
naturally become much separated after the latter had given rise to sieve- 
tubes by a stretching process. 
Oliver 2 describes the formation and obliteration of the pores of the 
sieve-plate in Macrocystis and Nereocystis. He describes and figures 
protoplasmic threads in the cross walls of the young sieve-tubes, and also 
gives some details of the formation of callus and slime-strings in the sieve- 
plates ; his paper, however, is chiefly concerned with the older stages 
in which callus has already begun to accumulate. He considers the callus 
masses which are formed on either side of the sieve-plates, and the long 
strings of callus which often entirely fill the lumen in the old trumpet 
hyphae, to be formations from the cell-wall, and he figures cases where the 
callus-stain is seen to fade into the wall, showing the change to be a 
gradual one. 
1 Rosenthal, 1 . c., 1890. 
3 Oliver, 1 . c., 1887, Figs. 89, &c., PL VIII. 
