IV. INTRODUCTION. 1 1 



cation with more success than among any other vegetables. In the subdivision of 

 AlgJB into the three groups of Chlorosperms, Melanosperms, and PJwdosperms, the 

 colour of the frond is, as we shall afterwards see, employed as a convenient diag. 

 nostic character. It is a character, however, which must be cautiously applied in 

 practice by the student, because, though sufficiently constant on the whole and 

 under ordinary circumstances, exceptions occur now and then ; and under special 

 circumstances Alga3 of one series assume in some degree the colour of either of the 

 other series. 



The green colour is characteristic of those that grow either in fresh water or in the 

 shallower parts of the sea, where they are exposed to full sunshine but seldom quite 

 uncovered by water. Almost all the fresh water species are green, and perhaps 

 three fourths of those that grow in sunlit parts of the sea ; but some of those of 

 deep water are of as vivid a green as any found near the surface, so thai we cannot 

 assert that the green colour is owing here, as it is among land plants, to a perfect ex- 

 posure to sunlight. Several species oiCaukrpa, Anadyomene, Codium.Bryopsis and 

 others of the Siphoneaj, which are not less herbaceous or vivid in their green colours 

 than other Chlorosperms, frequently occur at considerable depths, to which the 

 lio-ht must be very imperfectly transmitted. 



Algce of an olivaceous colour are most abundant between tide marks, in places 

 where they are exposed to the air, at the recess of the tide, and thus alternately 

 subjected to be left to parch in the sun, and to be flooded by the cool waves of the 

 returning tide. They extend however to low water mark, and form a broad belt of 

 vegetatio°n about that level, and a few straggle into deej^er water, sometimes into 

 very deep water. The gigantic deep-water Algaj, ilacrocystis, Ncreocystis, Lessonia, 

 and Durvillcea, are olive coloured. 



i?efZ-coloured Algaj are most abundant in the deeper and darker parts of the sea, 

 rarely growing in tide pools, except where they are shaded from the direct beams of 

 the sun either by a projecting rock, or by over-lying olivaceous Algte. The red 

 colour is always purest and most intense when the plant grows in deep water, 

 as maybe seen by tracing any particular species from the greatest to the least depth 

 at which it is found. Thus, the common Ceramium rubrum in deep pools or near 

 low-water mark is of a deep, full red, its cells abundantly filled with bright car- 

 mine endochrome, which will be discharged in fresh water so as to form a rose- 

 coloured infusion ; but the same plant, growing in open, shallow pools, near high Avater 

 mark, where it is exposed to the sun, becomes very pale, the colour fading through 

 all shades of pink down to dull orange or straw-colour. It is observable that this 

 plant, which is properly one of the red series (or Rhodosperms) does not become 

 o-rass-green (or like a Chlorosperm) by being developed in the shallower water, but 

 merely loses its capacity for forming the red- coloured matter peculiar to itself. So 

 also, Laurencia jrinnatijida, and other species of that genus, which are normally dark 

 purple, are so only when they grow near low water mark. And as many of them 

 extend into shallower parts, and some even nearly to high water limit, we find 

 specimens of these plants of every shade of colour from dull purple to dilute yellow 

 or dirty white. Similar changes of colour, and from a similar cause, are seen in 

 Chondrus crispus, the Carrigeen or Irish Moss, which is properly of a fine deep 



