PROGRESSIVE STAGES IN COMPLEXITY OF STRUCTURE. 587 



forth into the surrounding water, as in most water-plants belonging to this cate- 

 gory. The closed nets, on the other hand, are never joined to a substratum, but 

 remain floating in the water from which they derive their nourishment. 



The plate-like cell-complexes are composed of cells arranged in one plane, 

 and adjoining one another so as to leave no intercellular spaces. The partition- 

 walls inserted in the separate chambers during the development of this form 

 are arranged in two dimensions of space, and frequently intersect at right 

 angles. These cell-complexes either form a thin coating on stones or other 

 solid bodies, and then adapt themselves closely to all inequalities of the sub- 

 stratum, as e.g. in Protoderma viride, which covers the stones and old tree- 

 trunks in mountain - brooks ; or they appear as membranes, ribbons, and 

 delicate leaf-like structures, which are attached to the substratum only at one 

 point, and for the rest float freely in the water. This is what occurs in the 

 Sea Lettuce ( Ulva), and in many Floridese, as, for example, Porphyra. Sometimes 

 the plate-shaped complexes are developed as quite independent, unattached 

 tablets and discs, as in the genus Pediastrum (c.f. vol. II. fig. 197 6 ). The 

 leaf and ribbon-like forms which float in water are but seldom quite flat; 

 usually they appear much bent, undulated, and pitted; the margin, also, is 

 generally crinkled or slit, and divided into lobes and fringes, and these forms 

 thus furnish transitional stages, half cell-plates, and half cell-nets. In the matter 

 of size, all possible gradations are to be found, from the minute discs of Pedi- 

 astrum, and the small membranes of Prasiola flourishing in glacier-streams, up 

 to the Ulvas, living in the sea, many of which grow up into membranes a square 

 metre in area. 



Mass-like cell -complexes are those whose constituents adjoin one another 

 in three dimensions of space. Both in transverse and longitudinal sections 

 of their tissues we have at least two, but as a rule several stratified cell-layers. 

 Usually the whole body is elongated much more in one direction than in the 

 others; frequently it has the shape of a solid cylinder or prism, or it forms 

 thick fibres, cords, and ropes. Many remind one of earth-worms, or they 

 resemble the tentacles of polyps and sea -anemones. In many Florid eae, and 

 especially in the brown leathery sea -wracks, these cell -complexes are strap- 

 shaped, or they are contracted into a stalk below, where they are attached to 

 the substratum, and above widen out into leaf-like structures, as, for example, 

 in the Laminarias of the North Sea (see fig. 139), and in many other cases. 

 These strap-like, ribbon-like, and leaf-like structures occasionally remind one 

 of the similar plate-shaped cell-tissues of Ulvacese, previously mentioned, but 

 are distinguished from them by the fact that they are always built up of two 

 or more stratified layers of cells, so that a section taken at right angles to the 

 leaf-like structure always exhibits two cell-layers at least. Cake-like and ball- 

 shaped tissues are rarer. As examples of the latter various species of 

 Glceocapsa may be instanced, one of which is illustrated in fig. 25A, n. 



In most of these simple cell-complexes the bulk of the cells are shaped similarly. 



