6 rNTEODUCTION. 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 Algte 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 Algse proper leaves, like those of ph^nogamous 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 phyllodadia 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 Algaj, and first, 



OF THE EOOT. 



The root among the Algaj 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 Alg£e do not, 

 in a general way, derive nourishment from the soil on which they grow. We find 

 them growing indifi^erently 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 afibrds 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, eflfecting 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. Bache, 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 oi Hygrocrocis,* a group of Algae of low organization but strong diges- 



* Perhaps tlie Hi/grocrocis cuprica, Kiitz, or some allied species ; bixt I had no opportunity of examining a recent 

 specimen, and the characters cannot be made out from a dried one. 



