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 

 dificrent kinds. Thus sometimes the leaf, or phyUom,a, 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 lobed 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 Alga3 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 midi'ib 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 AVestpoint, 

 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 cell 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 ver}' 

 minute size, which by an expansion of its several parts will become a net like that 



VOL. ni. ART. 4. C 



