



464 



MR. T. JOHNSON ON THE SYSTEMATIC 



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cially on Dictyopteris polypodioides, and those of Reinke on the 

 Tilopteridea, and to give the Order a definite place in 



the 



Phteophycese. 









































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Structure of the Thallus. 

 The thallus of the Dictyotaceae is built up on a plan which is 

 essentially different from that on which the thallus of the 

 Florideae (excluding the Bangiaceae) is formed. In the Florideae, 

 according to Schmitz *, the thallus consists of a system of branch- 

 ing monosiphonous filaments, each filament growing by an apical 

 cell the segments of which never divide by median longitudinal 

 or median transverse walls, only by a gemmation of daughter- 

 cells which themselv es often become the apical cells of new fila- 

 ments. In the DictyotacesD, on the other hand, the thallus is 

 distinctly parenchymatous. The single apical cell, found through- 

 out life in Dictyota, and furnishing in D. dichotoma, as is well 

 known, the clearest example of dichotomy, is present at an early 

 stage in Taonia and Dictyopteris, being soon replaced, in Dicty- 

 opteris, by an initial group or marginal row of cells, the segments 

 of which divide by transverse and longitudinal walls, as in ordi- 

 nary parenchymatous cells. In Dictyopteris the thallus becomes 

 by a tangential division of the segments, generally two-layered 

 except in the region of the midrib, the production of which is, a* 

 Falkenberg f states, due to a repetition of localized tangential 

 divisions. It is of interest to note that the 8-10 layers of which 

 the midrib may consist are arranged very regularly (PI- XIII- 

 fig. 1), and not, as figured by Harvey, Kiitzing, and others, with- 

 out any order. Continuity of protoplasm may be shown, by 

 Hoffmann's blue and similar reagents, through the transverse 

 walls of the elongated central cells of the midrib and stalk. This 

 stalk in Dictyopteris is due to the disappearance from the basal part 

 of the midrib of the general lamina of the thallus. The transition 

 from the flattened midrib to the subcylindrical stalk is gradual. 

 Examination of the stalk by transverse and by longitudinal sections 

 shows that it grows in thickness, and is strengthened by the activity 

 of a meristematic layer, an incomplete cambial zone. The outer- 

 most layer, the epidermis or limiting tissue, is the meristematic 

 layer giving off, on its inner side only, by tangential divisions 



* 



* F. Schmitz, "Unters. ii. d. Befruchtung d. Florideen" (Sitz. d. k. Akad- 

 d. Wiss. z. Berlin, 1883 pp. 215-258), and Ann. Mag. Nat. Hist. 1884, vol. »«• 

 (ser. 5) pp. 1-29 and 80-96. 



1 Falkenberg, in Schenk'a Handb. ii. p. 232. 









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