ALGLE : THEIR STRUCTURE AND MODE OF REPRODUCTION. 33 
Macrocystis , and Lessonia , are distinguished by possessing 
zoospores, while the Fucacece are destitute of them. 
The structure of the various organs of the Fucacece may be 
studied in their most perfect form in our common Bladder- 
wrack, Fucus vesiculosus , one of the most abundant sea-weeds 
of our coast. It is found on all the shores of Europe, and in 
the North Sea it grows so abundantly that it is used for 
domestic purposes, such as roofing houses. It is also burnt, 
and alkali extracted from its ashes, the ordinary term 
64 potash ” being derived from this mode of obtaining it. A 
mature frond of this sea-weed, as shown in fig. 7, presents two 
conspicuous organs ; the bladders (6), which serve to float the 
fronds in the water; and the tubercles (<x), at the ends of the 
fronds, which contain the organs of reproduction. These repro- 
ductive organs are hollow bodies or conceptacles of two kinds, 
male and female ; in most of our common species the two 
kinds are found on different plants, which are therefore dioe- 
cious. These receptacles are not formed in the interior of the 
tissue, but as depressions of the surface which become walled 
in by the surrounding tissue, and so overgrown that at length 
only a narrow channel remains opening outwards. Among the 
reproductive organs a number of hairs are produced in the 
receptacles, which in Fucus platy carpus project out of the 
mouth of the receptacle in the form of tufts. The male con- 
ceptacles (fig. 8) produce the Antheridia as minute ovate sacs 
attached in great numbers to hair-like filaments. Each anthe- 
ridium consists of a thin- walled oval cell, the protoplasm of 
which splits up into a number of minute spermatozoids or 
antherozoids, pointed at one end, and endowed with sponta- 
neous motion caused by two vibratile cilia attached to each 
antherozoid ; in the interior they contain a red or orange 
speck. They are scarcely -^th of an inch in length ; the 
two cilia are of unequal length ; the larger one points forwards 
during progression, producing the motion, while the shorter 
one trails behind, and appears to act the part of a rudder. 
The female conceptacles (fig. 9) produce in the midst of the 
hairs a number of stalked oval bodies, the oogonia or spo- 
ranges. These bodies become filled with dark protoplasm, 
which divides into two, four, or eight spores, which are set 
free by the bursting of the envelopes or membranes in which 
they are confined. The process of fertilisation or fecundation 
of the spores has been carefully watched by M. Thuret, and is 
exceedingly curious. The process takes place outside the 
receptacles. At a certain period, when, from the reflux of the 
tide, these fertile branches are lying outside the water in moist 
air, the antheridia become detached from the hairs on which 
they grow, and collect in large numbers outside the mouth of 
VOL. XIII. — NO. L. D 
