10 
INTRODUCTION. 
IV. 
from which its parent cell was derived. Thus, supposing each cell of a single 
net of the Hydrodictyon were to be equally fertile, some myriads of new nets 
would be produced from every single net, as it broke up and dissolved. In this 
way a large surface of water might be filled with the plant in a single generation. 
The manner of growth of the frond is very various in the different families. In 
some, the body lengthens by continual additions to its apex, every branch being 
younger the further removed it is from the base ; that is, the tips of the branches 
are the youngest parts. This is the usual mode of growth in the Confervoicl 
genera, and also obtains in many of those higher in the series, as in the Fucaceie 
and many other Melanosperms. In the Lamina rite, on the contrary, the apex 
when once formed does not materially lengthen, but the new growth takes place at 
the base of the lamina, or in the part where the cylindrical stipe passes into the 
expanded or leaflike portion of the frond. In such plants the apex is rarely found 
entire in old specimens, but is either torn by the action of the waves, or thrown off 
altogether, and its place supplied by a new growth from below. In several spe- 
cies this throwing off of the old frond takes place regularly at the close of each 
season ; the old lamina being gradually pushed off by a young lamina growing 
under it. There are others, among the filiform kinds, in which the smaller branches 
are suddenly deciduous, falling off from the larger and permanent portions of the 
trunk, as leaves do in autumn from deciduous trees. Hence specimens of these 
plants collected in winter are so unlike the summer state of the species, that to a 
person unacquainted with their habits they would appear to be altogether different 
in kind. The summer and winter states of Rhodomela subfusca are thus different. 
In Desmarestia aculeata the young plants, or the younger branches of old plants, 
are clothed with soft pencils of delicate jointed filaments, which fall off when the 
frond attains maturity, and leave naked, thorny branches behind. Similar delicate 
hairs are found in many other Alga) of very different families, generally clothing the 
younger and growing parts of the frond ; and they seem to be essential organs, 
probably engaged in elaborating the crude sap of these plants, and consequently 
analogous to the leaves of perfect plants. This is as yet chiefly conjectural. The 
conjecture, however, is founded on the observed position of these hair-like bodies, 
which are always found on growing points, the new growth taking place imme- 
diately beneath their insertion. In most cases these hairs are deciduous, but 
in some, as in the genus Dcisya , they are persistent, clothing all parts of the frond 
so long as they continue in vigour. They vary much in form, in some being long, 
filiform, single cells ; in others, unbranched strings of shorter cells, and in others 
dichotomous, or, rarely, pinnated filaments. 
Three principal varieties of 
COLOUR 
are generally noticed among the Algae, namely, Grass-green or Herbaceous , Olive- 
green , and Red ; and as these classes of colour are pretty constant among otherwise 
allied species, they afford a ready character by which, at a glance, these plants may 
be separated into natural divisions ; and hence colour is here employed in classifi- 
