IV. INTRODUCTION. 3 



great series of the Alga9 at its lowest point, and proceeding upwards 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 

 Algaj, which renders the study of this portion of the vegetable world especiallj' 

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

 something of that general plan of development which nature has followed in con- 

 structing other and more compound plants, in which her steps are less easily traced. 

 From its first conception within the ovule to its full development, one of the higher 

 plants goes through transformations strictly analogous to stages of advancement 

 that can be traced among the Algas 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 successive stages of advancement. 



While their gradually advancing scale of development renders the study of these 

 plants more interesting, it also increases the difiiculty of constructing a short and 

 yet definite character, or diagnosis, which will include every member of the group^ 

 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 Alga3, 

 assuming that with the AlGjE are to be classed all Thallophytes (or Cryptogamic 

 plants destitute of proper axes, in the more restricted view of that term) which 

 ai'e developed in water, or nourished wholly through the medium of fluids, while 

 all Thallophytes that are ferial and not parasitic are Lichens, and all that are ajrial 

 and parasitic are Fungi. 



Commencing then with Algse of the simplest structure, a large part of them, 

 belonging to the orders Diatomacece and Desmidiacece, consist 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 membraneous coating of 

 the young plant (or cell), digested within its simple cavity, and the assimilated 

 matter applied to the extension of the cell-wall, until that has reached the size 

 proper to the species. Then the matter contained within the cavity gradually 

 separates into two portions, and at the same time a cell-wall is formed between 

 each portion, and thus the original simple cell becomes two cells. These no 

 longer cohere together, as cells do in a compound plant, but each half-cell separates 

 from its fellow, and commencing an independent cai-eer, digests food, increases in 

 size, divides at maturity, &c., going again and again through a similar round of 

 changes. In this way, by the process of self- division, and without any fructifi- 

 cation, a large surface of water may soon be covered with these vegetable monads, 

 from the mere multiplication of a single individual. 



These minute plants, (Diatomacece and Desmidiacece) from their microscopic size 

 and uniform and simple structure, are justly regarded as at the base of the vegeta- 



