It THE SMITHSONIAN INSTITUTION. 89 



less perfect, until the wtolc vegetable is reduced either to a root-like 

 body, or a branching naked stem, or an expanded leaf; as if Nature 

 had first formed the types of the compound vegetable organs so named 

 and exhibited them as separate vegetables ; and then, by combining 

 them in a single framework, had built up her perfect idea of a fully 

 organized plant. But among the Alg;G, we may go still lower in 

 vegetable organization, and arrive at ])lants where tlie wh(de body is 

 comi)osed of a few cells strung together ; and finally at others — the 

 simplest of known vegetables — whose whole framework is a single 

 cell. Tbese are the true vegetable monads: with these we commence 

 the great series of the Algaa at its lowest point, and proceeding iip- 

 wards we find, within the limits of this same series, all degrees of 

 complication of framework short of the development of proper flowers. 

 It is this progressive organization of the Algt\3 which renders the 

 study of this portion of the vegetable world especially interesting to 

 the philosophical botanist, because it displays to him, as in a mirror, 

 something of that general plan of development which Nature has fol- 

 lowed in constructing other and more compound ])lants, in Avhich her 

 steps are less easily traced. From its first conception within the ovule 

 to its full development, one of the higher plants goes tlirough transform- 

 ations strictly analogous to stages of advancement that can be traced 

 among the Alga3 from species to species, and from genus to genus, 

 from the least perfect to the most perfect of the group. Each Alga- 

 species has its own peculiar phase of development, which it reaches, 

 and there stops ; another species, passing this condition, carries the 

 ideal plan a step further ; and thus successive species exhibit succes- 

 sive stages of advancement. 



While their gradually advancing scale of development renders the 

 study of these plants more interesting, it also increases the difficulty 

 of constructing a short and yet definite character, or diognoais, which 

 will exclude every member of the grouj), and exclude species more 

 properly referable to the kindred groups of Lichens and Fungi. I 

 shall not here attempt any such critical definition, but proceed to trace 

 the gradual evolution of the frond and of the organs of fructification 

 in the Algaa, assuming that with the Alg^e are to be classed all Thallo- 

 phytes (or Cryptogamic plants destitute of proper axes, in the more 

 restricted view of that term) which are developed in water, or nour- 

 ished wholly through the medium of fluids, while all Thallophytes 

 that are atrial and not parasitic are Lichens, and all that are asrial 

 and parasitic are Fungi. 



Commencing then with Alga3 of the simplest structure, a large part 

 of them, belonging to the orders Diatomacece and Desmidiacece, con- 

 sist almost entirely of individual isolated cells. Each plant, or frond, 

 is formed of a single living cell ; destitute therefore of any special 

 organs, and performing every function of life in that one universal 

 organ of which its frame consists. The growth of these simple plants 

 is like that of the ordinary cells of which the compound frame of 

 higher plants is composed. Nourishment is absorbed through the mem- 

 branous coating of the young plant (or cell), digested within its sim- 

 jfle cavity, and the assimilated matter applied to the extension of the 

 cell-wall, until that has reached the size proper to tlie species. Then 

 the matter contained within the cavity gradually separates into two 



