FAMILY PALMELLEiE l PALMODICTYON. 



319 



detached from each other.* Of the third condition, we have an 

 example in the curious Palmodictyon described by Kiitzing ; the 



frond of which 

 Fig. 147. -r, appears to the 



naked eye like a 

 delicate network 

 consisting of 

 anastomosing 

 branches, each 

 composed of a 

 single or double 

 row of large vesi- 

 cles, within every 

 one of which is 

 produced a pair 

 of elliptical cel- 

 lules that ulti- 

 mately escape as 

 'Zoospores.' The 

 alternation be- 

 tween the 'mo- 

 tile ' form and 

 the < still ' or 

 resting form, 

 which has been 

 described as oc- 

 curring in Proto- 

 coccus (§ 189), 

 has been observed 

 in several other 

 forms of this 

 group j and it 

 seems obviously 

 intended, like the 

 production of 

 1 Zoospores,' to 

 secure the disper- 

 sion of the plant, 

 and to prevent 

 it from choking itself by overgrowth in any one locality. From 

 the close resemblance which many reputed Palmellacece bear to 

 the early stages of higher Plants (Fig. 147, a, b, c), considerable 

 doubt has been felt by many Naturalists whether they ought to be 

 regarded in the light of distinct and complete organisms, or whether 

 they are anything else than embryonic forms of more elevated 



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

 2nd Series, Vol. ii., 1848, p. 313), is one of fundamental importance in tlie 

 determination of the real characters of this group. 



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Successive stages of development of Viva. 



