SEA WEEDS 349 



The plant-body of a sea weed is called a thallus, and differs 

 considerably in the various species. Sometimes it has no expanded 

 portion whatever, but is more or less cylindrical in all parts, and 

 may be either branched or simple ; and in some species it forms a 

 simple crust or a soft jelly-like covering on a rock. 



All portions of a sea weed are made up of cells, and these are 

 never modified into vessels such as we see in the stems, leaves, and 

 roots of higher forms of vegetable life ; and one who is commencing 

 the study of the algse will find much interesting work in the exami- 

 nation of their microscopic structure. Thin sections of various 

 parts of the larger weeds, cut with a sharp knife or a razor, and 

 examined in a drop of water under a cover-glass, will show the 

 cellular structure perfectly ; while minute fragments of the small 

 and slender species are sufficiently thin and transparent to display 

 the form and arrangement of their cells without any previous pre- 

 paration. 



One of the principal charms of the marine algge lies in the great 

 variety of colour that they display. They all contain chlorophyll 

 that remarkable green colouring matter which enables a plant, 

 under the influence of light, to feed on the carbonic acid gas existing 

 in the atmosphere, or held in solution in water ; and with its aid 

 the sea weeds can utilise this product of decay and animal respira- 

 tion that would otherwise accumulate in the water of the sea. But, 

 in addition to this bright green chlorophyll, many of the sea weeds 

 contain a second colouring substance, and in these the great variety 

 of tint is dependent on the nature of the latter and on the proportion 

 in which it is present as compared with the chlorophyll itself. 



The different means by which the algse reproduce their kind 

 forms a most engrossing subject, and to the botanist a most impor- 

 tant one, for it has much to do with the classification of the species. 

 The affinities of plants may be better determined by the nature of 

 their reproductive processes than by any other features, but unfor- 

 tunately this is not so well understood with regard to the algae 

 generally as compared with many other divisions of the vege- 

 table kingdom ; and, as a consequence, there is still a considerable 

 difference of opinion, not only as to the extent of the whole group, 

 but also as to its divisions and subdivisions. The reason for this is 

 clear ; for while it is quite an easy matter to trace a flowering plant 

 through its complete cycle from seed to seed, it requires a much 

 more careful examination, combined with much microscopic work, 

 to trace a lowly organised plant from spore to spore. 



