988 . Transactions.— Botany. 
tion mingled with the mud at the bottom ;—others again are only discover- 
able by the microscope, or by the fact that, when they exist in vast numbers, 
they impart to the water a distinctive colour. But, in whatever situation 
or habitat these Alge are found, microscopic analysis reduces them all to 
the same elements as exist in the higher aerial plants—the vegetable cell— 
composed of an outer cellulose coat, a primordial utricle, and within this the 
coloured cell-contents, the endochrome, in which its vital activity is situated. 
A comparison of these subaqueous plants with their terrestrial congeners 
would form a most interesting subject of enquiry, but one of such vast 
dimensions that I can only venture to touch upon one or two of its most 
salient points this evening. 
Probably the typical form of the vegetable cellis a sphere. In all plants, 
however, except the very simplest—the unicellular—the spheres by aggre- 
gation become changed into various other figures, by mutual compression, 
and by their growing in the lines of least resistance. Thus we have the free 
globular cells of the Volvocinee; the cylindrical ones of the Conferve, the 
Zygqnemacee, ete., corresponding to the elongated cells of vascular and 
woody tissue ;—the quadrangular, polygonal, and irregular cells of Ulvacea, 
Pediastrea and Desmidiacee, which find their analogues in many parts of 
the epidermis, the expanded portion of leaves, the petals, ete., of the higher 
plants. Again the markings in dotted, spiral, and glandular vessels, are 
very similar in appearance, if in nothing else, to the markings in Lyngbye, 
Spirogyra, Calothrices, etc. It is singular to notice also, how, under some 
circumstances, the cell appears to endeavour to revert to its typical form, as 
in pl. XXIII., fig. 10, where the front view of = — —_* a — 
plex geometrical outline, and the side view exhi cel 
Into the question of the modes of combination of esis: det cells, a 
the exquisitely beautiful geometrical figures they often form, oF of the 
siliceous patterns secreted by the Diatoms, it is not my purpose to enter at 
present, though doubtless they have analogues in the shapes and forms of 
various flowers, and the arrangements of the elements of many leaf-buds. 
The colour of the endochrome of the fresh water Alge varies nearly a5 _ 
much as it does in flowering plants. In most it is green; in some, as the 
Oscillatoriea, it varies from light green through various shades of blue and 
purple to black; in the Protogocci again, we meet with different and often 
brilliant tints of red, and lastly in some Desmids and the majority of Diatoms 
shied by tw no exene of e rgele ng. = 
