60 
tliese of the lowest type, viz., the polycystina and sponges — the 
former investing the sarcodous mass with a silicious carapace, of 
marvellons beauty — the latter strengthening the horny matter with 
spicules of various shapes, or, as in Euplectella, weaving one of the 
hardest and most obdurate of substances into a framework of 
surpassing elegance. The elimination of silex is not confined to 
the protophyta; the grasses, canes, &c., do so, and deposit it in 
various forms on the exterior of the stems, or in the form .of scales 
on the leaves, and even the petals of the flowers. The Deutzia 
scahra is a familiar instance of this. Another substance, even 
more common than silica, viz., lime, does not, as far as I am 
aware, form any part of the structure of plants, from the lowest 
to the highest, excepting a genus of marine algse, the Corallina — and 
species of the genus Chara, are sometimes found covered with a 
calcareous crust, hut this is not essential to their well-being. 
Eaphides and sphaeraphides may also be cited as evidences of lime 
being taken up by plants, but these again appear to be of no im- 
portance to the welfare of the plant ; but if we turn to the animal 
kingdom, lime plays an all-important part in its economy — lime 
forms the humble home of the foraminifera, and it constitutes 
the principal portion of the mammal in its highest state of 
development. 
Ladies and gentlemen, I have endeavoured, as briefly as pos- 
sible, to convey to you some idea of what a diatom is, and its 
claims to a place in the vegetable kingdom, and in order to render 
this claim intelligible, I have thought it desirable to give a short 
account of the -\'egetable cell in its various forms ; and I think you 
will see that a desmid and a diatom are, if I may use the expres- 
sion, merely aiTested states of vegetable life, that is to say, the 
cells exist apart, and do not differentiate into various special 
organs, consequently the life of one cell does not necessarily 
depend on that of its neighbour. This is a pecuhar characteristic 
of the lower cryptogamia, for although many of the lower forms 
of flowering plants continue to live if cut into very small pieces, 
Anacharis alsinastrum, for example, the cells of which it is com- 
posed never exist apart ; but when we ascend higher in the scale 
of vegetable life, the cell is still less capable of self-existence, and 
special conditions have to be observed, to enable detached portions 
to vegetate : it will not do to cut indiscriminately — a piece of 
