rV. INTRODUCTION. O 



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 Alga3 the cell is cylindrical, and sometimes lengthened consider- 

 ably into a tliread-like body. Such is the formation of Oscillatorkv. In Vaicchericc 

 there is a further advance, the filiform cell becoming branched without any inter- 

 ruption to its cavity ; and such brandling 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 each 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 Palmclla and 

 Tetraspora we arrive at Ulva^ 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 Algaj 

 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 7iocIes, just as do the leaves and 

 branches of the higher compound plants. 



We have then two tendencies exhibited among Algte— 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 Algaj 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 saiTie individual, and 

 exhibited in greater or less perfection. We shall find stems becoming flattened at 



