Costario , Undaria , and Laminaria. 699 
is some time after the formation of all the ribs that we find the lamina of 
Costaria Turneri with an eroded apex. Griggs 1 states that Hedophyllum 
sessile measuring 2*3 mm. in length had already * several’ layers of cells in 
the middle region and towards the base, but with a monostromatic edge. In 
Lessoniopsis littoralis measuring about i-i mm. in length he also found its 
lamina already 4 several 5 layers of cells thick, even at the edge. Of the 
latter specimen he remarks: 4 It hardly seems possible that there could 
have been any remnant of the one-layered lamina which had not been trans¬ 
formed into the many-layered adult blade.’ In this respect he mentions 
Cymathere as the opposite case, 4 in which a large portion of the embryonic 
lamina is not changed, but continues to grow and persists until the plant 
is more than 20 cm. long.’ As regards the i-i mm. specimen of Lesso¬ 
niopsis he does not state whether the apex had been eroded or not. 
Judging, however, from his figure, the apical portion is clearly worn away. 
Still, his remark above quoted remains open to question unless the apex of 
the lamina had been proved to be persistent. Putting aside this question¬ 
able case as exceptional, there is ample reason to conclude that in Lamina- 
riaceae the monostromatic area of the blade is sooner or later atrophied 
without developing into complex tissue. 
Setchell 2 and McMillan 3 give the name 4 midrib ’ to the thickened area 
which runs longitudinally in the blades of Saccorhiza and Pterygophora 
respectively. In the present paper the area is mentioned under the name 
of meridional region in order to distinguish it from the true midrib. 
McMillan remarks that this area makes its first appearance in Pterygo¬ 
phora, when the plant has grown to several centimetres in height. So 
also in the case of Laminaria sp., as is described below. But in Co¬ 
staria Turneri , the primary midrib is first seen as a longitudinal eleva¬ 
tion in the polystromatic transitional region when a frond has attained 
1 cm. in the total height (Fig. 6). To the naked eye, however, it is not 
yet sharply defined from the adjacent region. Transverse sections through 
such points reveal the first step in the formation of the midrib as a few 
parenchymatous cells aggregated beneath the precortical layer of the ven¬ 
tral side. I mean by 4 ventral ’ the surface with the midrib elevated. The 
dorsiventrality of the lamina of Costaria is thus begun at an early stage 
of development. 
After the primary midrib has been formed and the frond has attained 
about cm. in length, the lateral ribs begin to come out, one on each 
half of the lamina. The new ribs are shorter than the primary one already 
in existence and are elevated on the dorsal surface. The three are parallel 
to one another for the greater part of their length, but gradually converge 
1 Briggs, 1 . c. 2 Setchell: Concerning the Life-history, See., p. 182. 
3 McMillan : Observations on Pterygophora . Minnesota Botanical Studies, 2nd series, Part VI, 
1902, pp. 725-6. 
