246 THE MICROSCOPE AND ITS REVELATIONS. 



tinuity with the cells, must be considered, as properly appertaining to 

 them. Sometimes these filaments radiate in various directions from a 

 tingle central cell, and must at first be considered as mere extensions of 

 this; their extremities dilate, however, into new cells; and when these are 

 fully formed, the tubular connections close up, and the cells become 

 detacned from each other. 1 Of the third condition, we have an example 

 in the curious Palmodictyon described by Kutzing; the frond of which 

 appears to the naked eye like a delicate network consisting of anastomos- 

 ing branches, each composed of a single or double row of large vesicles, 

 within every one of which is produced a pair of elliptical cellules that 

 ultimately escape as 'zoospores.' The alternation between the i motile' 

 form and the i still ' or resting form, which has been described as occur- 

 ring in Protococcus ( 231), has been observed in several other forms of 

 this group; and it seems obviously intended, like the production of 

 t zoospores/ to secure the dispersion of the plant, and to prevent it from 

 choking itself by overgrowth in any one locality. It is very commonly 

 by plants of this group, that the Algal portions of Lichens are formed 

 ( 325). 



244. 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 de- 

 finite membranous expansion. The mode in which this frond is pro- 

 duced may be best understood by studying the history of its develop- 

 ment, some of the principal phases of which are seen in Fig. 144; for the 

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

 a Protococcus, give rise by their successive subdivisions in determinate 

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

 Converfoid 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 development), the filament increases in 

 breadth as well as in length (as seen at E), and finally becomes such a 

 6 frond ' as is shown at F, G. In the simple membranous expansion, or 

 thallus, thus formed, there is no approach to a differentiation of parts by 

 even the semblance of a formation of root, stem, and leaf, such as the 

 higher Algae present; 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 ex- 

 hibit an imperfect partitioning into four parts, preparatory to multipli- 

 cation by double subdivision; and the entire frond usually shows the 

 groups of cells arranged in clusters containing some multiple of four. 



245. Besides this continuous increase of the individual frond, how- 

 ever, we find in most species of Ulva a provision for extending the plant 

 by the dispersion of ' zoospores;' for the endrochrome (Fig. 145, a) sub- 



! This fact, first made public by Mr. Thwaites ("Ann. of Nat. Hist.," 3d 

 Series, Vol. ii., 1848, p. 313), is one of fundamental Importance in the determina- 

 tion of the real character of this group. 



