320 ULVACE.E I DEVELOPMENT AND MULTIPLICATION. 



types. On this question great light has been thrown by the re- 

 cent observations of Dr. Hicks, who has shown it to be almost 

 certain that a large proportion (to say the least) of these so-called 

 Unicellular Alga? are really the gonidia of Lichens. * 



240. Notwithstanding the very definite form and large size 

 attained by the fronds or leafy expansions of the Ulvacece, to which 

 group belong the grass-green Sea-weeds (or 'Lavers') found on every 

 coast, yet their essential structure differs but very little from that 

 of the preceding group ; and the principal advance is shown in this, 

 that the cells, when multiplied by binary subdivision, not only 

 remain in firm connection with each other, but possess a very 

 regular arrangement (in virtue of the determinate plan on which 

 the subdivision takes place), and form a definite membranous 

 expansion. The mode in which this frond is produced may be best 

 understood by studying the history of its development, some of the 

 principal phases of which are seen in Fig. 147 ; for the isolated 

 cells (a), in which it originates, resembling in all points those of a 

 Protococcus, give rise, by their successive subdivisions in determi- 

 nate directions, to such regular clusters as those seen at B and c, 

 or to such Confervoid filaments as that shown at d. A continuation 

 of the same regular mode of subdivision, taking place alternately 

 in two directions, may at once extend the clusters b and c into 

 leaf -like expansions ; or, if the filamentous stage be passed through 

 (different species presenting variations in the history of their de- 

 velopment), the filament increases in breadth as well as in length 

 (as seen at b), and finally becomes such a frond as is shown at p, G. 

 In the simple membranous expansions thus formed, there is no ap- 

 proach to a ' differentiation' of parts by even the semblance of a 

 formation of Root, Stem, and Leaf, such as the higher Alga? pre- 

 sent ; every portion is the exact counterpart of every other ; and 

 every portion seems to take an equal share in the operations of 

 growth and reproduction. Each cell is very commonly found to 

 exhibit an imperfect I partitioning into four parts, preparatory to 

 multiplication by double subdivision ; and the entire frond usually 

 shows the groups of cells arranged in clusters containing some 

 multiple of four. 



241. Besides this continuous increase of the individual frond, 

 however, we find in most species of Ulva a provision for extending 

 the plant by the dispersion of ' Zoospores ;' for the Endochrome 

 (Fig. 148, a) subdivides into numerous segments (as at b and c), 

 which at first are seen to lie in close contact within the cell that 

 contains them, then begin to exhibit a kind of restless motion, and 

 at last pass-forth through an aperture in the cell-wall, acquire four 

 or more cilia (d), and swim freely through the water for some time. 

 At last, however, they come to rest, attach themselves to some fixed 



,; See his admirable Memoirs in " Quart. Journ. of Microsc. Science," 

 Vol. viii. (18G0), p. 239, and Vol. i. N.S. (1861), pp. 15, 90, 157. 



