IV. 
INTRODUCTION. 
9 
slimy, and in others it has nearly the firmness of cartilage. On the degree of its 
compactness and abundance depends the relative substance of the plant ; which 
is membranaceous where the gelatine is in small quantity ; gelatinous where it is 
very abundant and somewhat fluid ; or cartilaginous where it is firm. 
The frond may be either cylindrical or stem-like, or more or less compressed and 
flattened. Often a cylindrical stem bears branches which widen upwards, and 
terminate in leaf-like expansions, which are of various degrees of perfection in 
different kinds. Thus sometimes the leaf, or phylloma , is a mere dilatation ; in 
other cases it is traversed by a midrib, and in the most perfect kinds lateral nerve- 
lets issue from the midrib and extend to the margin. These leaves are either 
vertical, which is their normal condition, or else they are inclined at various angles 
to the stem or axis, chiefly from a twisting in their lamina, the insertion of 
the leaf preserving its vertical position. They are variously lobecl or cloven, and 
in a few cases (as in the Sea Colander of the American coast) they are regularly 
pierced, at all ages, with a series of holes which seem to originate in some portions 
of the lamina developing new cells with greater rapidity than other parts, thus causing 
an unequal tension in various parts of the frond, and consequently the production of 
holes in those places where the growth is defective. Such plants, though they form 
lace-like fronds, are scarcely to be considered as net works. Net-like fronds are, 
however, formed by several Algre where the branches regularly anastomose one 
with another, and form meshes like those of a net. Most species with this 
structure are peculiar to the Southern Ocean, but in the waters of the Caribbean 
Sea are found two or three which may perhaps yet be detected on the shores of 
the Florida Keys. In one of the Australian genera of this structure ( Claudea ) 
the net-work is formed by the continual anastomosis of minute leaflets, each of 
which is furnished with a midrib and lamina. The apices of the midribs of one 
series of these leaves grow into the dorsal portion of leaves that issue at right 
angles to them, and as the leaves having longitudinal and horizontal direc- 
tions, or those that form the warp and weft of the frond, are of minute size and 
closely and regularly disposed, the net-work that results is lace-like and delicately 
beautiful. 
In the Hydrodictyon , a fresh water Alga, found in ponds in Europe and in 
the United States, where it was first detected by Professor Bailey near Westpoint, 
a net-like frond is formed in a different manner. This plant when fully 
grown resembles an ordinary fishing-net of fairy size, each pentagonal mesh being 
formed of five cells, and one cell making a side of the pentagon. As the plant 
grows larger, the meshes become wider by the lengthening of the cells of which 
each mesh is composed. When at maturity, the matter contained within 
each ceil of the mesh is gradually organised into granules, or germs of fu- 
ture cells, and these become connected together in fives while yet contained 
in the parent cell. Thus meshes first, and at length little microscopic net- 
works, are formed within each cell of the meshes of the old net ; and this 
takes place before the old net breaks up. At length the cells of the old net 
burst, and from each issues forth the little network, perfectly formed, but of very 
minute size, which by an expansion of its several parts will become a net like that 
vol. ni. art. 4. c 
