172 STRUCTURE OF THE THALLUS OF DELESSERIA SANGUINEA. 



the vein, with the exception of the most external layer, are much 

 longer than broad and have their protoplasm continuous with each 

 other and with those cells which touch them (fig. 5). 



As regards the stalk we find that the outer layer is made up of 

 small squarish cells, while scattered irregularly in the centre are very 

 large cells between which smaller cells are packed (fig. 6). Fig. 7 

 shows that the large cells are conducting cells, and that their proto- 

 plasm is continuous at their ends and also with the neighbouring 

 cells at the sides. 



We now come to consider the physiological importance of these 

 structures, and at the outset we can clearly distinguish between an 

 assimilating tissue, viz. the parts of the leaf where it is only one cell 

 thick and the outermost cells covering the veins, and a conducting 

 tissue, namely, the veins in the leaves and large cells in the stalk. 

 The cell wall is thick, gelatinous, and pitted, and by means of these 

 pits the protoplasm is continuous from cell to cell. The assimilated 

 substances can easily be passed through a few cells to the veins ; here 

 they find an easy passage to various parts of the plant, some being 

 stored up in the stalk which serves for a reservoir of reserve material 

 as well as for an organ of attachment. Comparing the Delesseria 

 leaf with that of a Dicotyledon, in both cases we have conducting 

 tissue, but in the Delesseria there is only one conducting tissue re- 

 quired to take the assimilated substances to paints of the plant where 

 they are wanted, and since the plant lives entirely submerged there 

 is no need of a tissue to conduct water to the leaf ; and we see too 

 that in Delesseria, coupled with morphological differentiation into stalk 

 and leaf, there is also physiological differentiation into assimilating 

 and conducting tissue. 



The above remarks apply especially to D. sanguinea, and judging 

 from Harvey's pictures we conclude that in the main all the species 

 are alike in possessing assimilating and conducting tissue. 



DESCRIPTION OF PLATES XVII and XVIII. 



Illustrating Mr. M. C. Potter's paper " On the Structure of the 

 Thallus of Delesseria sanguinea." 



Fig. 1. — Part of thallus of Delesseria sanguinea. 



Fig. 2. — Portion of leaf magnified, a. The midrib, h. Lateral veins given off from it. 

 c. Fine veins. 



Fig. 3. — Transverse section of leaf. A. Section showing the unicellular part (a) and a 

 small vein (b). B and C. Section showing the development of a vein. 



Fig. 4. — Portion of a leaf seen in surface view. 



Fig. 5. — Cells from a fine vein. 



Fig. 6. — Transverse section of portion of stalk, the shaded portions (a) indicating the 

 pits where the cells are continuous at their ends. 



Fig. 6. — Longitudinal section of stalk, showing a large conducting cell. 



