THE PH^OPHYCEAN ZOID. 

 By a. H. Church. 



Considered as a ciliated reproductive cell, the characteristic 

 "zoospore" or "swarm-spore" of the Phseophyceaj presents little 

 interest ; but regarded from a broader standpoint as the retention 

 of a flagellated phase in the life-cycle, linking this great group of 

 marine algae with other flagellated races of phytoplankton, it acquires 

 an intensive value — not only as opening up the question of the 

 flagellate ancestry of the Phseophycese, but as expressing the high- 

 gi-ade differentiation attained by such a zoid in some previous phase 

 of existence and its isolated phyletic history. 



The organization of the Phseophycean zoid as presented most 

 typically in the zoogonidia of Ectocarpus, the gametes of Laminaria^ 

 or the antherozoid of Ficcus, is that of a simple naked protoplast, 

 rounding off at about 5 fx diam., with nucleus, suggestions of granular 

 cytoplasm and either a single discoid chloroplast reduced to an eye- 

 spot {stigma) only, or associated with such a residual plastid ; larger 

 zoids may contain several chloroplasts. Such a zoid, though typically 

 possessing autotrophic mechanism, differs in one fundamental respect 

 from a typical flagellate, in that it has lost the power of binary 

 fission and is so far retained wholly for a " reproductive " function. 

 On the other hand, many '* Brown Flagellates " are similarly re- 

 stricted to binary fission in the non-motile "cyst "-stage (cf, Hyclru- 

 rus, PhcBosphcera, PJiceocapsa, Hymenomonas). 



The zoid is thus to be regarded from the standpoint of the 

 organization of a flagellate, though such details have been as yet but 

 little investigated. 



Though generally described as pyriform in shape, with pointed 

 anterior end. the protoplast possesses little in the way of a permanent 

 space-form ; when at rest the body rounds off in response to surface- 

 tension ; and in the motile stage the pointed end is largely the 

 expression of " metabolic " or '* euglenoid " contractility ; in this way 

 a 5 /z individual may extend to 7-10 or 12 ^ in length, as an elon- 

 gated zoid with active movements. The point of insertion of the 

 fiagella must be regarded as the '*;?oZ<9" of the zoid; and in such 

 case the direction of the movement implies a change of polarity, of 

 about 90'^, from the original condition of the isokont phase with equal 

 distally inserted fiagella. 



A similar change of polarity obtains in other flagellate phyla, noticeably 

 in many Cryptomonads (cf. Nephroselmis), and in the Peridiniaceae, with a 

 very similar result ; and this is undoubtedly correlated with a differentiation 

 in function between the two fiagella, as expressed in a reversal of the direc- 

 tion of contractility in one which becomes a propeller, while the other 

 remains a tractor. With two fiagella thus widely divergent, and falling into 

 line at 180° with each other across the axis of antecedent polarity, both act 

 in the same direction, and a new " anterior " end is acquired. 



Details of zoid organization in terms of flagellate construction are 

 still meagre : — 



Yamanouchi (1913) for Zanarduiia described a suggestive chain 

 Journal of Botam. xA.peil, 1919. [Slpplemem Jl.] 



