ALGM. 209 



grains (grains of chlorophyll), which are always present when cells are combined 

 into • tissues. The configuration of the cell-wall is much less varied than in 

 other classes of plants, at least in so far as it depends on increase of thickness, 

 stratification, and chemico-physical differentiation. In general a tendency prevails 

 towards the transformation of the cell-wall into mucilage; and this process not 

 unfrequently plays a most important part in the setting free of the reproductive 

 cells. Lignification, on the other hand, occurs very rarely, perhaps never ; greater 

 firmness is attained only by deposition of silica (in the Diatoms), or of calcium 

 carbonate (Acetabularia, IMelobesiaceae). Most of the Algae of which the cells are 

 united into tissues are ductile, flexible, and slimy. Sometimes the cell-wall (or 

 single layers of it) is brightly coloured; its increase in thickness is generally 

 almost uniform over the outside of the cell, but strong projections are often directed 

 outwards. A localised increase in thickness on the inner side of the cell-wall is 

 observable only in cells combined into tissues {e.g. Fucacese, Florideae) in the 

 formation of simple dots. A decided differentiation of the cell-wall into layers 

 differing from one another in chemico-physical properties often occurs in the resting- 

 spores {e. g. Vaucheria, CEdogonium, Spirogyra). The cuticularisation of the outer 

 layers never advances far inwards, the cuticle generally remaining thin. 



The Mode of Combifiaiio?i 0/ the Cells with one another is more various among 

 Algae than in any other class of plants. In the most simply organised forms, 

 where the vegetative cells which belong to one cycle of development are nearly 

 alike, their combination into a tissue appears useless; they therefore not unfre- 

 quently separate from one another by division when first formed, and live isolated 

 (so-called Unicellular Algae). But in other cases, where the cells also show no 

 perceptible diversity, they remain combined into tissues, either as rows (filaments) 

 or plates, or masses of cells, according to the direction of growth and of the 

 divisions perpendicular to this direction. Two modes of tissue-formation occur 

 among the more lowly organised Algae ; — the enclosure of one cell within another, 

 and the subsequent union of cells previously free. In both cases the cells form 

 one family; in the former case, such as we find in Gloeocapsa and Gloeocystis, 

 each cell, before it divides, forms a thick watery cell-wall, so that the second genera- 

 tion of cells appears as if enclosed in the cell-wall of the first generation, the third 

 in that of the second generation, and so on. Sometimes the thick mucilaginous 

 cell-walls run into one another in such a manner that their boundaries can no 

 longer be distinguished (Volvocineae); in other cases a row of cells in genetic 

 connexion (a linear cell-family) forms in this manner an envelope of jelly around 

 itself, or several coalesce into a larger lump of jelly, in which the strings of cells 

 are imbedded (Nostoc). Less frequent is the union and subsequent common 

 growth of cells previously isolated which have been formed by the division of 

 one mother-cell; such a union forms a plate, as in Pediastrum, or a hollow 

 net, as in Hydrodictyon. Larger masses of tissue not unfrequently arise in 

 Algce by the common growth of numerous rows or filaments of cells lying close 

 to one another, and by their division in a corresponding manner ; single filaments 

 may in this manner become intimately combined {Coleochcete scufata), or may lie 

 in loose juxtaposition {ColeochcBie soluta). A tissue-like structure may even result 

 from one and the same cell becoming variously branched, and its ramifications 



p 



