6 
INTRODUCTION. 
IV. 
their summits into leaves, and leaves, by the loss of their lateral membranes, and 
the acquisition of thicker midribs, changing into stems ; and among the most highly 
organized Algfe we shall find leaf-like lateral branches assuming the form, and to a 
good degree the arrangement of the leaves of higher plants. Not that we find 
among Algae proper leaves, like those of phaenogamous plants, constantly deve- 
loping buds in their axils ; for even where leaf-like bodies are most obvious (as 
in the genus Sargassum ), they are merely phylloclaclia or expanded branches ; 
as may readily be seen by observing a Sargassum in a young state, and watching 
the gradual changes that take place as the frond lengthens. These changes will be 
explained in the systematic portion of this work. 
I shall now notice more particularly the varieties of habit observed among the 
compound Algae, and first, 
OF THE ROOT. 
The root among the Alga; is rarely much developed. Among higher plants which 
derive their nourishment from the soil in which they grow, and in Fungi which 
feed on the juices of organized bodies, root-fibres, through which nourishment is 
absorbed, are essential to the development of the vegetable. But the Algae do not, 
in a general way, derive nourishment from the soil on which they grow. We find 
them growing indifferently on rocks of various mineralogical character, on floating 
timber, on shells, on iron or other metal, on each other, — in fine, on any substance 
which is long submerged, and which affords a foothold. Into none of those substances 
do they emit roots, nor do we find that they cause the decay, or appropriate to them- 
selves the constituents, of those substances. They are nourished by the water that 
surrounds them and the various substances which are dissolved in it. On those 
substances they frequently exert a very remarkable power, effecting chemical changes 
which the chemist can imitate only by the agency of the most powerful appa- 
ratus. They actually sometimes reverse the order of chemical affinity, driving out 
the stronger acid from the salts which they imbibe, and causing a weaker acid to 
unite with the base. Thus they decompose the muriate of soda which they absorb 
from sea-water, partly freeing and partly appropriating the chlorine and hydrogen ; 
and the soda is found combined in their tissues with carbonic acid. 
A remarkable instance of the action of a minute Alga on a chemical solution was 
pointed out to me by Prof. Baclie, as occurring in the vessels of sulphate of copper 
kept in the electrotyping department of the Coast Survey office at Washington. 
A slender confervoid Alga infests the vats containing sulphate of copper, and 
proves very destructive. It decomposes the salt, and assimilates the sulphuric acid, 
rejecting (as indigestible !) the copper, which is deposited round its threads in a 
metallic form. It sometimes appears in great quantities, and is very troublesome ; 
but the vats had been cleaned a few days before I visited them, so that I lost the 
opportunity of examining more minutely this curious little plant. Most probably 
it is a species of Hygrocrocis , * a group of Algae of low organization but strong diges- 
* Perhaps the Ilygrocrocis citprica, Kiitz, or some allied species ; but I had no opportunity of examining a recent 
specimen, and the characters cannot be made out from a dried one. 
