560 MICROSCOPIC FORMS OF VEGETABLE LIFE THALLOPITYTES 



Protococcus, 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 presenting variations in the history of their develop- 

 ment), the filament increases in breadth as well as in length (as seen 

 at E), and finally becomes such a ' frond ' as is shown at F, G. In 

 the simple membranous expansion or thallus thus formed, there is 



but little approach to a 

 differentiation of parts in 

 the 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 opera- 

 tions of growth and repro- 

 duction. Each cell is very 

 commonly found to exhibit 

 an imperfect partitioning 

 into four parts preparatory 

 to multiplication by double 

 bipartition, 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 



Si! KM :;3S!/ ft frond, however, we find, in 



most species of Utoa, a 

 provision for extending the 

 plant by the dispersion of 

 zoospores. The endochrome 

 (fig. 425, 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 escape by the bursting of the 

 cell- wall, and swim freely through the water as zoospores (d) by 

 means of their flagella, each zoospore having become endowed 

 with either two or four flagella during its formation within its 

 mother-cell. At last, however, they come to rest, attach them- 

 selves to some fixed point, and begin to grow into clusters or 

 filaments (e) in the manner already described. The walls of the 

 cells which have thus discharged their endochrome remain as 

 colourless spots on the frond ; sometimes these are intermingled with 



FIG. 424. Successive stages of development 

 of Ulva. 



