vol. xxiv.]

Note on an Alga (Dermatophyton radicans, Peter) growing on the European Tortoise. By M. C. POTTER, M.A., F.L.S., Assistant Curator of the University Herbarium, Cambridge.

[Read 2nd June, 1887.]

(PLATE VIII.)

THE alga which is the subject of this paper is found growing principally upon the dorsal surface of the carapace of the water tortoise (Clemmys caspica), which inhabits the southern parts of Europe, and, by spending a great part of its existence in water, offers its back as a suitable nidus for algæ.

The alga appears to the naked eye as irregular but roundish dark-green patches which vary very much in size, often having a diameter of about a quarter of an inch, as at a, fig. 1, Pl. VIII. On

removing a patch and cutting sections in a direction perpendicular to the animal's back, it is found to be composed of numerous rather large cells arranged very close together and generally square in shape. Some of these cells are exposed to the action of the water, and form a plate of cells a few layers thick, closely applied to the tortoise-shell, and others are found as wedge-shaped masses which penetrate into the shell. The cells of the plate whose free surface is exposed to the action of the water continually divide in directions perpendicular and parallel to the surface of the tortoise-shell; the outermost layer continually forms zoospores, and so prevents the plate from becoming more than a few cells thick. The cells next to the tortoise-shell are closely adpressed to it, and individually have the power of penetrating into any crack of the tortoise-shell which may present itself to them. When an algal cell meets with a crack, it strives to penetrate into it, in doing which it opens the crack more and more and so penetrates further in, and as it does so it divides first by a plane perpendicular to its direction of growth and then by planes perpendicular and parallel to this direction, and so forms wedge-shaped masses of cells, as at a in figs. 2 and 3, which grow and penetrate in any direction in which they can force open the crack. Sections cut parallel to the animal's back show on the outside masses of algal cells where the thick ends of wedges are cut through (a, fig. 3), and towards the centre sections of thinner parts of wedges (b and c, fig. 3). Thus a section through an algal patch similar to fig. 2, parallel to the animal's back, would be represented by a drawing resembling fig. 3.

If sections of the tortoise-shell and alga are allowed to remain in water for some few days, it is found that not only does the alga remain alive but continues to grow healthily. The cells exposed to the water continually form zoospores, while the layers in contact with the tortoise-shell, since the surrounding pressure is removed, tend to grow out into filaments (a and b, fig. 4). These filaments can be formed by any cells touching the tortoiseshell; they have a very irregular shape, and the chlorophyll is always situated at the growing end.

As before mentioned, the alga is reproduced by means of zoospores which are formed from the outermost layers of cells. The cell about to form a zoosporangium becomes flask-shaped, a kind of neck being formed, the contents of the cell dividing into a considerable number of zoospores, which have the usual pyriform