lO THALLOPHYTES. 



-coming attached to one another in such a manner (as in Acetabularia or Udotea) 

 lat a section of the whole shows a number of cell-cavities, which however are all 

 lerely bulgings of a single cell. But none of these structures, which either actually 

 institute or only imitate a cellular tissue, have any great importance in the vege- 

 ,ble kingdom, being limited to single groups of Algse ; while aggregations of cell- 

 laments will be met with more completely developed among Fungi and Lichens, 

 'he more highly developed forms of Algae, which at the same time have the power 

 f forming large individuals, — as the various species of Fucus, Laminaria, Sargassum, 

 id some Florideae, — exhibit the usual mode of the formation of tissue which here 

 lakes its appearance for the first time in the vegetable kingdom, and is met with 

 I a continuously increasing grade of development in Muscineae and Vascular 

 [ants. The successive segments of the apical cell lying at the end of each branch 

 f the thallus first form a primary meristem which then passes over into more or less 

 Lverse forms of permanent tissue, although these different forms show only slight 

 idications of that specialized internal structure which we find partially even among 

 le Muscinese, and universally among Vascular plants. The whole tissue forms a 

 ind of parenchyma, the cells of W'hich, however, are everywhere densely compact, 

 tcept where single large air-cavities are to be formed. A differentiation of this 

 3sue into different systems is usually only so far indicated that the outermost layers 

 insist of smaller and firmer cells, while the inner cells are often very large, and 

 )metimes extremely long (as in some Floridese). 



The External Differentiation into Organs is only feebly indicated in the lowest 

 .Igse ; but, if we go through the different series of forms in the class, it passes through 

 le most varied gradations. It is first manifested by the growth following the direction 

 f one axis only, by which an elongated sac (Vaucheria) or, with the intervention of 

 sll-division, a row^ of cells is formed. In this case the growth may either take place 

 t all points (intercalary as in Spirogyra), or may be limited to a terminal cell, or 

 lay gradually cease at a greater distance from it. The external differentiation attains 



higher grade when branching takes place ; at first this is uniform, the branches 

 ^sembling the axis in their mode of growth ; in more highly differentiated forms 



loses its uniformity, the lateral shoots developing in a manner different to the axis 

 'hich produces them. Both may occur in a single cell, a row of cells, or a solid 

 ssue. The external diflerentiation of the Algae, as far as it is manifested in the 

 Drmation of an axis with lateral outgrowths arranged according to certain laws, 

 hows in its higher stages a differentiation which distinctly calls to mind the differ- 

 nce between stem and leaf, and to a certain extent even the formation of roots. In 

 tie genus Caulerpa these relationships are strikingly evident, although the whole 

 hallome consists only of a single cell; it becomes differentiated into a creeping 

 tem with leaf-like lateral branches arising in acropetal order, and root-like filaments 

 ;rowing downwards ; and a similar differentiation occurs to a smaller extent even in 

 .^aucheria. Much more varied is the differentiation of the lateral branches in those 

 hallomes which consist of numerous cells (solid tissues), since they produce in 

 lefinite order below their growing apex leaf-like appendages, or at places root-like 

 ihoots (Fucacese, the larger Floridese, &c.). While the leaf-like members of the 

 hallome may not be clearly distinguishable morphologically from actual leaves, the 

 oot-like members are at the same time always distinguishable from true roots as 



