THE SMITHSONIAN INSTITUTION. 97 



Rhodomela suhfusca are thus different. In Desmaresiia aculeafa the 

 young plants, or the younger branches of ohl phants, are clothed with 

 soft pencils of delicate jointed filaments, wliicli i'all oft" when the frond 

 attains maturity, and leave naked, thorny branches behind. Similar 

 delicate hairs are found in many other Alg;e of very different families, 

 generally clothing the younger and growing parts of the frond ; and 

 they seem to be essential organs, probably engaged in elaborating the 

 crude sap of these plants, and consequently analogous to the leaves of 

 perfect plants. This is as yet chiefly conjectural. The conjecture, 

 however, is founded on the observed position of these hair-like bodies, 

 which are always found on growing points, the new growth taking 

 place immediately beneath their insertion. In most cases these hairs 

 are deciduous ; but in some, as in the genus Dasya, they are persist- 

 ent, clothing all parts of the frond so long as they continue in vigor. 

 They vary much in form, in some being long, filiform, single cells ; 

 in others, unbranched strings of shorter cells, and in others dichoto- 

 mous, or, rarely, pinnated filaments. 

 Three principal varieties of 



COLOR 



are generally noticed among the Algaa, namely. Grass-green or Her- 

 baceous, Olive-green, and Med; and as these classes of color are 

 pretty constant among otherwise allied species, they afford a ready 

 character by which, at a glance, these plants may be separated intO' 

 natural divisions ; and hence color is here employed in classification 

 with more success than among any other vegetables. In the subdi- 

 vision of Alg;e into the three groups of Chlorosperms, MelanospermSy 

 and Pihodosperms , the color of the frond is, as we shall afterwards see,, 

 employed as a convenient diagnostic 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 cir- 

 cumstances Alga3 of one series assume in some degree the color of 

 either of the other series. 



The green color 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 that we cannot as- 

 sert that the green color is owing here, as it is among land plants, 

 to a perfect exposure to sunlight. Several species of Caiderpa, Ana- 

 dyomene, Codium, Bryopsis and others of the Siphoneas, which are not 

 less herbaceous or vivid in their green colors than other Chloro- 

 sperms, frequently occur at considerable depths, to which the light 

 must be very imperfectly transmitted. 



AlgJB of an olivaceous color 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, how- 

 ever, to low-water mark, and form a broad belt of vegetation about 

 7 



