14 BRITISH SEA-WEEDS. 



As the present arrangement of the lied weeds is 

 founded on the structure of the spore-clusters, it is very 

 important that the student should become thoroughly 

 acquainted with these organs. In the various species of 

 such genera as Poly sip Jionia, Laurencia, etc., they may 

 be easily examined under a microscope of moderate 

 power, by simply pressing the conceptacle between two 

 slips of glass, and in most of the more perfect divisions 

 of the series they may be exhibited either in longitudinal 

 or transverse sections of the conceptacles, Among the 

 species of the less perfect section the process of examina- 

 tion is more difficult, but a little practice and perseve- 

 rance will generally yield sufficient results to answer the 

 purpose of all ordinary collectors. 



The tetr as pores are formed either from the bark-cells 

 or from the upper branchlets. They are very various^ 

 arranged in different genera. In some they are scat- 

 tered singly among the surface-cells over the whole 

 frond, in others they are collected in more or less dis- 

 tinct clusters {sort) ; in others, again, they are found 

 only in the branchlets; or in external warts (nemathe- 

 cia); or in special leaflets {sporophylla) ; or in pod-like 

 receptacles [stichidia). They are always produced in a 

 constant regular manner, in accordance with fixed na- 

 tural laws, and are all constructed on the same principle. 

 Each consists of a dark-coloured mass of endochrome, 

 enveloped in a transparent, membranous sac, and marked 

 by lines for division into four parts. This mass, when 

 mature, separates into four, or rarely into eight or more 

 spondes. The divisions into four — whence the name 

 of tetraspore, which is derived from the Greek tetras, 

 four, and spoils a seed — are variously made in different 

 genera,, and are frequently used as distinctive charac- 



