10 INTEODUCTION. IV. 



•from whicli 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 Confervoid 

 genera, and also obtains in many of those higher in the series, as in the Fucacete 

 and many other Melanosperms. In the Laminarise, 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 beloAV. 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 

 tinder 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 autunan 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 Rliodo7nela 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 Algae 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 Dasya^ 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 Algce, namely. Grass-green or Herbaceous, Olive- 

 green^ and Bed ; 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- 



