CONFERVOIDEJE ISOGAM^ 



273 



marked as in the unicellular Multinucleatse. The class includes one 

 large order, the Confervacea, and three smaller ones, the Ulotrickacece, 

 Pithophoracea, and ChroolepidecB, though the boundaries between them 

 are not in all cases well defined. In the Confervacese and Ulotrichacese 

 the filament which springs from the germination either of a zoospore 

 or of a zygosperm resulting from the conjugation of zoogametes, attaches 

 itself to the substratum — a stone, another alga, or sorne other aquatic 

 plant — by a rhizoid, which may consist of a single cell, or may branch 

 into a number of cells. As in the higher algae, the rhizoid is not a 

 nutritive organ, but simply an organ of attachment. These algae may, 

 however, continue to grow and retain their vitality for a long period in 

 water without any attachment to the substratum. 



Order 1. — Confervace^ {including ChjEtophorace^). 



The term Confervaceae has been very 

 vaguely applied to a variety of green fresh- 

 water organisms, but is now limited to a 

 comparatively small number of genera of 

 fresh-water, and a few brackish and salt- 

 water alg«, in which each individual 

 ■consists of a segmented branched or un- 

 branched filament of cylindrical or disc- 

 shaped cells, invested by a mucilaginous 

 ■sheath, and in which multiplication takes 

 place non-sexually by megazoospores, or 

 sexually by the conjugation of smaller 

 zoogametes. Both kinds of swarm-cell have 

 two cilia, or the former in some cases 

 four ; Lagerheim describes, in Conferva 

 bombycina (Ktz.), megazoospores with a 

 single cilium. From each parent-cell are 

 produced either one or two megazoospores. 



In only a few species has the process 

 of conjugation of zoogametes been actually 

 observed, and the systematic position of a 

 large number of the species is therefore 

 at present only conjectural. Areschoug has 

 followed both the conjugation of the zoo- 

 gametes and the direct germination of the 

 megazoospores in Urospora (Aresch.). In 

 -Conferva (L.), Chsetophora (Schr.), Draparnaldia (Ag.), and some other 



T 



Fig. 242. — Microspore JIoccQsa Thur. 

 ^, ^, portions of filament. C, fila- 

 ment dividing for the escape of zoo- 

 spores. Z*, zoospores ( x 300). 

 (After Cooke.) 



