IY. 
INTRODUCTION. 
3 
great series of the Algae 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 
Algae, 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 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 Algae 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 difficulty 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 Algie, 
assuming that with the Alghs are to be classed all Thallophytes (or Cryptogamic. 
plants destitute of proper axes, in the more restricted view of that term) which 
are developed in water, or nourished wholly through the medium of fluids, while 
all Thallophytes that are as rial and not parasitic are Lichens, and all that are aerial 
and parasitic are Fungi. 
Commencing then with Algae of the simplest structure, a large part of them, 
belonging to the orders Diatomaceoe 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 career, 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- 
