310 



MICROSCOPIC FORMS OF VEGETABLE LIFE. 



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 regular membranous stratum. The mode in which this 

 frond is produced, is best understood by studying the history of 

 its development, some of the principal phases of which are seen 

 in Fig. 104 ; for the isolated cells (A), in which it originates, 

 resembling in all points those of a Proiococcus, give rise, by their 

 successive subdivisions in determinate 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 present- 

 ing variations in the history of their development), the filament in- 

 creases in breadth as well as in length (as seen at E), and finally be- 

 comes such a frond as is shown at F, G. In the simple membranous 

 expansions thus formed, there is no approach to a " differentia- 

 tion" of parts, by even the semblance of a formation of root, stem, 

 arid 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 exhibit an imperfect partitioning 



FIG. 105. 



Formation of Zoospores in Phycoseris gigantea (Ulva latissima) ; a, portion of the ordinary frond: 

 fc, cells in which the endochrome is beginning to break up into segments; c, cells from the boundary 

 between the colored and colorless portion, some of them containing zoospores, others being empty ; 

 d, ciliated zoospores, as in active motion ; e, subsequent development of the zoospores. 



into four parts, preparatory to multiplication by double subdivi- 

 sion ; and the entire frond usually shows the groups of cells 

 arranged in clusters containing some multiple of four. Besides 

 this continuous increase of the individual frond, however, we 



