PALMELLACEJE ; ULVACE.E 



559 



have an example in the curious Palttwdictyon described by Kiitzing, 

 the frond of which appears to the naked eye like a delicate network, 

 consisting of anastomosing 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 zocispores. The 

 alternation between the motile form and the still or resting form, 

 which has been described as occurring in Protococcus, has been ob- 

 served in several other forms of this group ; and it seems obviously 

 intended, like the production of 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. 1 



Notwithstanding the very definite form and large size attained 

 by the fronds or leafy expansions of the Ulvacese, to which group 



FIG. 423. jHcematococcus sanguineus, in various stages of development ; <(, single 

 cells, enclosed in their mucous envelope ; I, c, cluster formed by subdivision of 

 the parent-cell; d, more numerous cluster, its component cells in various stages of 

 division; e, large mass of young cells, formed by the subdivision of the paivn! 

 endochrome, and enclosed within a common mucous envelope. 



belong some of the most common grass-green seaweeds ('laver') 

 found on every coast, yet their essential structure (lifters but very 

 little from that of the preceding group ; and the principal advance- 

 is shown in this, that the cells, \vlu-n multiplied by binary sub- 

 division, 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 mem 

 branous 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. 424. The isolated 

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



'- [The Pal iiii'llin-i'ic are not now regarded by the best authorities as a distinct 

 family from the Prutococcaceic, and the genus Htei/iatococcus is sunk in Proto- 

 coccus. ED.] 



