508 Transactions. 
Bostrychia arbuscula the outer layer is composed of thick-walled polygonal 
cells containing olive granules. In Cystophora dumosa there are one or 
periphery, and with moderately thick cell-walls. Within this layer is 
large-celled parenchyma. Lessonia variegata has on each side of the thallus- 
lamina three or four rows of minute densely aggregated cells with clear 
walls and olive-brown granules. Within these is a transparent tissue of . 
large cells, and in the centre a band of mechanical tissue. A dense outer 
Po LAC 
( f) Water-reservoirs in Algae.—'Two species of brown algae living well 
up in the intertidal belt have a hollow thallus filled with a watery sub- 
stance. In Hormosira Banksii the internodes are swollen and filled with — 
_ (g.) Covering of Slime or Mucilage.—Many algae have a covering of 
slime on the outside. Splachnidium rugosum and Codium adhaerens are 
exceedingly slimy. Ulva and Porphyra possess a thin coat, which on drying 
is slightly sticky to the touch. A covering of slime is frequent in many 
marine animals, е. 
SEA-WATER. 
‚ (a.) Buoyancy.—As with land-plants, algae are so constructed as to 
display their vegetative structure as much as possible to the surrounding - 
medium and to the light. On account of the movement of the water, rigid | ar. 
tissue is inadmissible in the littoral belt except for very-low-growing forms 
(Corallina, Melobesia). Devices for keeping algae upright in the water 
the thalli, or large air-cells. The air-vesicles vary in shape and position. T 
In Cysptophora, Marginaria, and Carpophyllum they are globose and ter- 
te short branc n Macrocystis pyrifera it is the stalks of the 
air-cells are best seen in Durvillea antarctica, where they are polygonal — 
and Se stretching across the frond at right angles to the pi a 
amina. 
; › ; 8 | 
Invariably begins on the apex of gasteropods or umbones of pelecy] a 
shells—that is, on those parts which are longest exposed to the action % == — 
the air or water. To strengthen the parts of the shell thus worn -— di 
