9G TENTH ANNUAL REPORT OF 



one of tlie Australian genera of tliis structure (Claudea) the net-work 

 is formed by the continual anastomosis of minute leaflets, each of 

 which is i■urni^hed with a midrib and lamina. The apices of the mid- 

 ribs 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 longitudi- 

 nal and horizontal directions, 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 West Point, a net-like frond is formed in a dilterent man- 

 ner. This plant when fully grown resembles an ordinary fishing-net 

 of fairy size, each pentagonal mesh being formed of five cells, and one 

 ceil 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 comi)osed. When at maturity, the matter contained within 

 each cell of the mesh is gradually organized into granules, or germs 

 of future 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 net-work, perfectly formed, but of very minute size, which, by 

 an expansion of its several parts, will become a net like that 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 pdant 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 ot growth in the Confervoid genera, and also ob- 

 tains in many of those higher in the series, as in the Fucacea; and 

 many other Melanosperms. In the Laminariai, 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 speci- 

 mens, but is either torn by the action of the waves or thrown off alto- 

 gether, and its place supplied by a new growth from below. In sev- 

 eral species 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. Plence specimens of these 

 plants collected in winter are so unlike the summer state of the spe- 

 cies, that to a person unacquainted with their habits they would appear 

 to be altogether different in kind. The summer and winter states of 



