OF NORTHUMBERLAND AND DURHAM. ^ 275 



layer the vasa mucifera are placed ; tliey are large and very easily 

 seen (figs. 9, 10). 



L.flexicaulis. — The stem is flaccid, compressed to within a very 

 short distance of the rootj and is always perfectly smooth and 

 polished. A transverse section is almost plano-convex in out- 

 line, the under surface of the stem being generally much flat- 

 tened. The " pith" appears merely as a narrow curved line, 

 corresponding to the long diameter of the section, and curving 

 downwards at each extremity towards the plane surface. There 

 are no vasa mucifera, and no separable cortex, the whole stem 

 consisting of a simple cellular tissue, increasing gradually in 

 density from the pith to the epidermis (figs. 1, 2, 3), and there is 

 never any appearance of concentric zones, such as are seen in 

 L. Cloustoni. The rooi-/Z>re5 are disposed irregularly. The frond 

 is destitute of any dense cellular axis, and its vasa are so small 

 as to be generally undistinguishable in a transverse section 

 (fig. 4). 



But, in addition to structural peculiarities, there are other 

 remarkable differences between the two species. L. Cloustoni 

 grows always in deep water, being constantly out of reach, even 

 at the lowest spring tides ; its stem is rigid enough to support 

 itself easily in an erect position, even when removed from the 

 water, and is often covered with parasitic Alga^ and zoophytes, 

 its roughness probably enabling these to attach themselves 

 readily. All parts of the plant abound with a thick, viscid 

 mucus, which exudes copiously from the cut or fractured sur- 

 faces when steeped in fresh water. L. flexicaulis, on the 

 other hand, is found abundantly between tide marks, grow- 

 ing generally in a horizontal position, on the sides of deep 

 rock pools, or amongst loose stones, w^here it is left dry 

 and prostrate on the recess of the tide. It is scarcely ever 

 infested by parasites of any kind. The stem of L. Cloustoni 

 is perennial ; the frond is annual, and is renewed every 

 spring, so that at that time of the year it is scarcely pos- 

 sible to find a plant which does not exhibit some stage of the 

 new growth. L. flexicaulis is altogether perennial — at least no 

 renewal of the frond has hitherto been observed. 



