3 1 8 Sykes. — A nato7ny and Histology of Macrocystis pyrifera and 
sieve-tubes together with the primary pith filaments represent the whole of 
the young thallus, the entire cortex of the adult being of later origin. 
Growth in thickness in Macrocystis , as in Laminaria , is due to the 
production pf this late cortex by the divisions of an outer cortical meristem, 
and no other meristem is ever present. When the primary cortex has been 
entirely transformed to sieve-tubes, - there is no reason to suppose that 
further sieve-tubes are produced from the secondary cortical cells in either 
plant. Hence the increase in size of the medulla, after additional sieve-tubes 
have ceased to be formed, must be ascribed to the swelling of the walls 
and the greater diameter of the elements already present, also to the 
numerous hyphae which grow in from the inner cortical layers and form an 
anastomosing system throughout the medulla. 
In Laminaria saccharina the innermost secondary sieve-tubes, produced 
from the primary cortex, have much swollen walls, and resemble in every 
particular the primary pith filaments. On the other hand, even in the 
oldest stem examined, the outermost secondary sieve-tubes remain more or 
less rudimentary, but there can be no doubt that both kinds of elements 
are to be regarded as homologous with the well-known sieve-tubes 
of Macrocystis. In the latter, however, the secondary sieve-tubes have 
attained a far greater degree of development ; they differ in size and general 
appearance from the primary pith filaments, and there is no gradual 
transition as in Laminaria. This difference in Laminaria may well be 
explained as due to the more gradual growth in the latter plant, there being 
no pause in the differentiation of the elements of the original thallus, but 
merely a slow diminution in amount of elongation, from the central cells to 
those of the periphery. It is probable that the greater development of the 
sieve-tubes in Macrocystis is due to its habit of growth and to the consider- 
able length attained by the stem, as has already been suggested by Oliver 1 
and Wille 2 . 
In longitudinal section it is of interest to compare the appearance of 
an adult stem of Laminaria with that of a young plant of Macrocystis in 
the stage described as Stage ii 3 . At that age it is found that there are only 
two or three layers of sieve-tubes as yet developed from the primary cortex, 
while the still rapidly elongating square-ended cells of the remainder of the 
cortex are not yet transformed into sieve-tubes. While the differentiated 
sieve-tubes are here to be compared with those of the adult Laminaria 
which become part of the medulla, the primary cortical cells are in 
appearance not unlike the outermost sieve-tubes in Laminaria , which 
always remain rudimentary. 
The main points in connexion with the development of the sieve-plate 
in the two plants are strikingly similar, but some difference in detail 
is found in Macrocystis which may be largely due to the far greater size 
1 Oliver, 1 . c., 1887. 3 Wille, 1 . c., 1897. 3 p. 299. 
