INTRODUCTION. 7 



uncommon, and are well worthy of a place in every col- 

 lection. 



The fronds of some of the filiform genera are deci- 

 duous, and their branchlets fall off like leaves in Au- 

 tumn, causing the same kind of difference between the 

 summer and winter aspect of the plant as there is be- 

 tween a tree covered with leaves, and one whose branches 

 are bare. Several species of Rhodomtla and Polysipho- 

 nia have fronds of this description. 



There are other genera, the young fronds of which 

 are clothed with tufts of delicate, jointed fibres, that fall 

 off as the frond becomes mature. These fibres, which 

 are most strikingly obvious in Desmarestia, Sporochnus, 

 and Arthrocladia, but exist in variously modified forms 

 in genera widely separated by other characters, are al- 

 most always found on the growing part of the frond, and 

 it is therefore supposed that their functions are similar 

 to those of the leaves of higher plants. 



One other organ that I must notice is the air-vessel, 

 or float, which occurs almost exclusively among plants 

 of the olive series. It assumes various forms, but is 

 always adapted to increase the buoyancy of the stem 

 and branches, and so to lessen the resistance which 

 they offer to the advancing or receding waves. In some 

 genera these vessels are simple cavities, formed by swell- 

 ings of the stem or branch, and filled with air, as in 

 the common Popweed {Fucus nodosus). In others, seve- 

 ral placed near together merge into a single, chambered 

 vessel, as in Halidrys siliquosa. In some of the Sar- 

 gassa they are borne, berry-like, at the tips of short 

 branches. This is notably the case with the Gulf- weed, 

 which derives its specific name of bacciferum, or berry- 

 bearing, from these organs. In addition to those forms 



