clusters at the ends of branches, the oogonia usually solitary glo- 

 bose; antheridia 1 to several in the cluster, formed at the tip of the 

 branch, becoming cut off by a septation, producing numerous motile 

 antherozoids. Large akinetes are common. 

 There is but one species in this genus. 



Dichotomosiphon tuberosus (A. Braun) Ernst 1902, p. 115 



PL 68, Figs. 6,7 

 Thallus a dichotomously branched tube with constrictions at the 

 base of the branches and with cross walls only where reproductive 

 structures are cut off; forming felt-like or cushion-like mats in the 

 silt of lake bottoms, and consisting of horizontal, downwardly 

 projecting, and erect, sex-organ-bearing branches; much enlarged, 

 elongate, subcylindric or globose akinetes frequent; sex organs 

 monoecious, located at the ends of strongly recurved branchlets 

 which form corymb-like clusters at the ends of vegetative filaments; 

 usually 1 spherical oogonium and several cylindrical antheridia 

 present; the oospores globular, becoming yellow at maturity and 

 easily seen with the unaided eye, oospore wall 3-layered; vegetative 

 siphons 50-100/x in diameter; oogonia 250-300/x in diameter; oospore 

 nearly the same size as the oogonium; antheridia 35-50/a in diameter, 

 2-4 times the diameter in length. 



This plant seems to grow only in lakes with a rich organic silt. 

 The thallus is often nearly buried in loose bottom deposits with 

 only the tips of vertical branches and the sex organs emerging. In 

 Wisconsin the plant has not been collected except from hard water 

 or eutrophic lakes. In Michigan it is common at depths up to 16 

 meters, but in such habitats it appears to reproduce only by akinetes, 

 sex organs being formed when the plant grows at 2 meters or less. 

 This may be related to water temperature. Like Vaiicheria, 

 Dichotomosiphon forms a habitat for an association of microfauna 

 such as Rotifera, Cladocera, and many protozoa. 



From several lakes, mostly from silty bottoms. Mich., Wis. 



VAUCHERIA De Candolle 1805, p. 61* 

 Thallus subaerial or aquatic, a much and irregularly branched 

 siphonous coenocyte without cross walls except where reproductive 

 structures are cut off; filaments often compactly interwoven (espe- 

 cially in terrestrial species) to form a felt-like expansion, with 

 colorless rhizoids when attached, or forming entangled clots of 

 coarse threads when floating; branches arising laterally or dichoto- 

 mously, nearly as large as the main filament and tapering slightly 

 to broadly rounded tips; chloroplasts numerous ovate discs without 



"See footnote, p. 288. 



[290] 



