IV. 
INTRODUCTION. 
11 
cation with more success than among any other vegetables. In the subdivision of 
Algas into the three groups of Chlorosperms , Melanosperms , and fihodosperms , 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 Algae 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 groAv 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 that 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 of Caulerpa , Anadyomene , Coclium, Bryopsis and 
others of the Siphoneaa, which are not less herbaceous or vivid in their green colours 
than other Chlorosperms, frequently occur at considerable depths, to which the 
light must be very imperfectly transmitted. 
Algse 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 
vegetation about that level, and a few straggle into deeper water, sometimes into 
very deep water. The gigantic deep-water Algas, Macrocystis, Nereocystis, Lessonia , 
and Durvillaea , are olive coloured. 
JiAi-colourcd Algas 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 Algae. The red 
colour is always purest and most intense when the plant grows in deep water, 
as may be 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 water 
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 PJiodosperms) does not become 
grass-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 pinnatijida , 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 
