Micractinium pusillum var. elegans G. M. Smith 1918, p. 631 



PI. 66, Fig. 7 

 A variety diflFering from the typical by having more numerous 

 and longer setae on the free walls of the cells; setae 5-7 in 

 number. 



Rare; in plankton. Wis. 



Micractinium quadrisetum (Lemm.) G. M. Smith 1916, p. 479 



PI. 68, Fig. 1 

 A free-floating colony of 4 ovate cells, adjoined to other groups 

 of 4 about a central rectangular space; wall with 1-4 very long, 

 finely tapering setae; chloroplast a parietal cup with 1 pyrenoid; 

 cells 4-7 /A in diameter, 8-10/* long; setae 23-40ju, long. 

 Rare; in the plankton of lakes. Mich., Wis. 



ORDER SIPHONALES 



In this order the thallus is a coenocytic tube, more or less branched 

 and usually showing a basal-distal differentiation. In marine forms 

 the thallus becomes very elaborate and complex; in the fresh-water 

 genera the vegetative thallus is simplified, although sexual repro- 

 duction is of an advanced type. There are no cross walls except where 

 sex organs or zoospores are cut off, although in Dichotomosiphon 

 there are frequent constrictions of the filament. There are numerous 

 ovate chloroplasts peripherally arranged in a layer of cytoplasm 

 along the wall, the center of the tube being vacuolate. There are 

 no pyrenoids, but starch is stored in Dichotomosiphon, and either 

 starch or oil may accumulate in Phijllosiphon and Vaucheria.^ 



Asexual reproduction is accomplished by zoospores or aplano- 

 spores. In sexual reproduction a high type of heterogamy is involved. 

 Enlarged oogonia, each with a single egg, are cut off by a wall 

 from the vegetative filament, usually near tubular antheridia which 

 produce large numbers of small biflagellate antherozoids. Fertiliza- 

 tion is effected through a terminal pore in the wall of the oogonium. 



Key to the Families 



Plants endophytic phyllosiphonaceae 



Plants free-living, either aquatic or subaerial vaucheriaceaf 



FAMILY PHYLLOSIPHONACEAE 



In this family the thallus is a much contorted tubular or vesicular 

 coenocyte which is either endozoic or (in our specimens) endophytic. 

 The plants inhabit the tissues of the Araceae, especially in tropical 

 and subtropical climates. There is but a single genus in our region. 



'Now considered a member of the Chrysophyta. 



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