Laminaria saccharina . 
295 
Neither does Oltmanns’ 1 account of the production of the sieve-tubes 
in Macrocystis help to clear up the difficulty. 
No definite secondary growth in thickness has ever been described in 
any other Laminariaceae except Nereocystis and Thalassiophyllum. The 
former appears to agree closely with Macrocystis 2 , both in its mode of 
development and in its adult anatomy, but the latter, described by 
Rosenthal 3 , has an inner cortical meristem which takes the place of the 
original outer cortical meristem, the latter soon ceasing to function. 
iii. Morphological nature of the various elements composing the medulla 
in the Laminariaceae. The terminology of the various elements composing 
the medulla has been a matter of some difficulty. 
(a) It is fairly agreed that the large outer tubes in Macrocystis and 
Nereocystis are probably true sieve-tubes (Oltmanns, Oliver, Will, Wille, 
Rosenthal), but it is uncertain whether the original pith cells should be 
included in this category. These have been termed ‘ trumpet hyphae ’ by 
Oliver, but Oltmanns 4 has recently pointed out that, whether they are to 
be looked upon as sieve-tubes or not, they are certainly not hyphae. The 
latter suggests that probably both they and the large sieve-tubes will be 
found to possess sieve- plates, whose manner of formation will prove to be 
similar to that of the true sieve-plates in the Phanerogams. 
Wille 5 also considers the ‘sieve-cells’ or central pith cells, both of 
Laminaria and of Macrocystis , to be homologous with the larger sieve- 
tubes of Macrocystis ; whilst Oliver, on the other hand, maintains that they 
are of an entirely different nature, and are not to be regarded as true sieve- 
tubes. He lays stress on the fact that he found no cross connexions in 
Macrocystis between the pith-cells, or ‘ trumpet hyphae and the true 
sieve-tubes. 
Oliver 6 and Wille 7 both bring forward the suggestion that the presence 
of these large sieve-tubes in Macrocystis and Nereocystis is correlated with 
the habit of these plants. 
Oltmanns, however, suggests that their function may be in part 
mechanical. 
(/3) Hyphae. Various elements in the medulla have been termed hyphae 
by different authors, but it seems generally agreed that this loose use of the 
term is not to be commended, and that only the ingrowing filaments, arising 
from the cortex, can be really looked upon as true hyphae. It is as yet 
uncertain whether these form secondary connexions. 
iv. Mucilage canals. The mucilage canals in Macrocystis and other 
Laminariaceae have been carefully described and figured by Will 8 and 
others, and do not immediately concern this account. 
1 Oltmanns, 1 . c., p. 452. 2 Macmillan, 1 . c., 1897. Oliver, 1 . c., 1887. 
3 Rosenthal, 1 . c., 1890. 4 Oltmanns, 1 . c., pp. 453-454. 5 Wille, 1 . c., 1897. 
6 Oliver, 1 . c., 1887, p. 112. 7 Wille, 1 . c., 1897. 8 Will, 1 . c., 1884. 
