288 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 Algae 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 cell is 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 Volvocinew, the cylindrical ones of the Conferim, the 

 ZygnemacecB, etc., corresponding to the elongated cells of vascular and 

 woody tissue ; — the quadrangular, polygonal, and irregular cells of Ulvacece, 

 Pediastrece and DesmidiacecB, which find their analogues in many parts of 

 the epidermis, the expanded portion of leaves, the petals, etc., 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 Lyngbym, 

 Svirorjyra, 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 pi. XXIII., fig. 10, where the front view of the pediastrum shows a com- 

 plex geometrical outline, and the side view exhibits four simple circular cells. 



Into the question of the modes of combination of these algal cells, and 

 the exquisitely beautiful geometrical figures they often form, or 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 Algse varies nearly as 

 much as it does in flowering plants. In most it is green ; in some, as the 

 OscillatoriecB, it varies from light green through various shades of blue and 

 purple to black ; in the Protococci again, we meet with different and often 

 brilliant tints of red, and lastly in some Desmids and the majority of Diatoms 

 with a reddish or yellowish brown hue, although the endochrome of many 

 Diatoms is, in early life, of a brilliant green colour. Taking the fresh-water 

 Algse altogether, and comparing them with the leaves and flowers of the 

 aerial plants, there appears to be a strong resemblance between the colours 

 exhibited by these two extremes of the vegetable kingdom. The various 



