171 



On the Structure of the Thallus of Delesseria 

 sanguinea. Lamour. 



By 

 M. C. Potter, M.A., F.Ii.S. 



With Plates XVII and XVIII. 



The Delesserias, a genus o£ red seaweeds, six species of wliicli in- 

 habit the shores of the British Isles, have the thallus differentiated 

 into a well-marked foliar expansion and a cylindrical portion; these 

 we may respectively term the leaf and the stalk. Descriptions of this 

 thallus are given in all books on seaweeds, for instance, in Harvey^s 

 (Phycologia Britannica), or Hauck's (Die Meeresalgen), but as far as 

 I am aware no detailed account has been given of these structures. 

 Agardh"^ and Willet briefly describe their histology, but pay no 

 special attention to them. The leaf is generally a fairly broad expan- 

 sion (PI. X"VIII, fig. 1), with a definite midrib which gives off laterally 

 a number of veins, these again giving off other veins and so on ; the 

 veins taper and become gradually finer till they end in being uni- 

 cellular (fig. 2). This arrangement of veins exactly resembles the 

 distribution of the fibro-vascular bundles in the leaf of an ordinary 

 Dicotyledon and we shall see that they perform nearly the same func- 

 tions in the two cases. By cutting transverse sections of the leaf we 

 find that it is a plate of cells one cell thick (fig. 3, A, a) with here and 

 there masses of cells, the veins. The cells of the leaf seen in surface 

 view are polygonal with their protoplasm continuous J from cell to cell 

 (fig. 4). While the leaf is small and is still growing the veins are 

 developed by the cells of the leaf dividing into three (fig. 3, B) ; the 

 cells on each side may divide similarly and thus the vein become 

 broader (fig. 8, C) ; the cells also above and below (fig. 8, A, b) the 

 central ones may cut off segments and thus the vein becomes thicker. 

 In this way we can trace the development of a vein from its unicellular 

 ending to its largest part where it joins the midrib or another large 

 vein. The midrib is developed in exactly the same way. The cen- 

 tral cells first formed (fig. 3, C) elongate considerably as the young 

 leaf grows ; the cells formed by the divisions of the outer ones do 

 not become so long as the central cell ; thus the cells in the centre 

 of each vein are longer than those external to them. The cells of 



* Florideernes Mor^phologi, Koiigl. Svenska Veteiiskaps Akademiens Handlingar, 1879. 

 f Bidrag til Algernes Physiologiske Anatomi, Kgl. Svenska Vetensk. Akad., 1885. 

 X On the Continuity of the Protoplasm in the Floridecp, see Gardiner, in Proc. Camb. 

 Phil. Soc.j vol. V, p. 104, and Hick, iu Journal of Jiotany, vol. xxii. 



