IV. 
INTRODUCTION. 
5 
pronounce on this question, because it does not immediately concern our present 
subject, and because, in all its collateral bearings, it requires more attentive exami- 
nation than it has yet undergone. 
In Protococcus the cell of which the plant consists is spherical or oval ; in other 
equally elementary Algm the cell is cylindrical, and sometimes lengthened consider- 
ably into a thread-like body. Such is the formation of Oscillator ice. In Vauclierice 
there is a further advance, the filiform cell becoming branched without any inter- 
ruption to its cavity ; and such branching cells frequently attain some inches in 
length, and a diameter of half a line, constituting some of the largest cells known 
among plants. 
In all these cases eacli cell is a separate individual : such plants are therefore 
the simplest expression of the vegetable idea. But even in this extremest sim- 
plicity we find the first indication of the structure which is to be afterward evolved. 
Thus in the spherical cell we have the earliest type of the cellular system of a com- 
pound plant developing equally in all directions ; and in the cylindrical cell, the 
illustration of the vertical system developing longitudinally. These tendencies, 
here scarcely manifest, become at once obvious when the framework begins to be 
composed of more cells than one. 
Thus in the genera nearest allied to Protococcus , the frond is a roundish mass 
of cells cohering irregularly by their sides. From these through Palmella and 
Tetraspora we arrive at TJlva, where a more or less compact membranous expan- 
sion is formed by the lateral cohesion of a multitude of roundish (or, by mutual 
pressure, polygonal) cells originating in the quadri-partition of older cells ; that is, 
by the original cells dividing longitudinally as well as transversely, thus forming 
four new cells from the matter of the old cell, and causing the cell-growth to pro- 
ceed nearly equally in both directions. Starting, therefore, from Protococcus , and 
tracing the developement through various stages, we arrive in Ulva at the earliest 
type of an expanded leaf. 
In like manner the earliest type of a stem may be found by tracing the Algae 
which originate in cylindrical cells. Here the new cells are formed in a longi- 
tudinal direction only, by the bipartition of the old cells. Thus, in Conferva , 
where the body consists of a number of cylindrical cells, strung end to end, these 
have originated by the continual transverse division of an original cylindrical cell. 
Such a frond will continually lengthen, but will make no lateral growth ; and con- 
sisting of a series of joints and interspaces, it correctly symbolizes the stem of one 
of the higher plants, formed of a succession of nodes and internodes. And the 
analogy is still further preserved when such confervoid threads branch; for the 
branches constantly originate at the joints or nodes , just as do the leaves and 
branches of the higher compound plants. 
We have then two tendencies exhibited among Algae — the first, a tendency to 
form membranous expansions, the symbols or types of leaves ; the second a ten- 
dency to form cylindrical bodies or stems. Among the less perfect Algae the whole 
plant will consist either of one of these foliations, or of a simple or branched stem. 
But gradually both ideas or forms will be associated in the same individual, arid 
exhibited in greater or less perfection. We shall find stems becoming flattened at- 
