482 LIFE-HISTORIES 



single nucleus and a chromatophore in the form of a nearly 

 complete hollow cylinder. 



Eventually in some of the cells (B) the protoplasm assumes a 

 spheroidal form or may divide into from two to eight smaller masses 

 each provided with a nucleus through division of the original one. 

 These globular masses soon begin to move and presently make their 

 way into the surrounding water through an opening in the old 

 cell-wall. When outside, however, they are still surrounded by a 

 delicate cellulose meml)ranc, but this soon ruptures setting free 

 the naked protoplasts. Each of these (C) is now seen to be some- 

 what pear-shaped, with a colorless pointed end from which come four 

 slender lash-like projections, called flagcUaA The rounded part is 

 grass-green and contains a bright red granule termed the eye-spot. 

 As soon as they are free, these naked protoplasts swim about with 

 rapid motion, propelled by their lashing flagella. After a while they 

 come to rest, secrete a cellulose wall, and germinate b}^ fission, the 

 lo-wer one of the two cells first formed becoming the pseudo-root by 

 elongation and attachment to the substratum, while the upper 

 cell develops into a long green multicellular thread by repeated 

 divisions. A naked motile protojilast, by means of which a plant 

 is multiplied non-sexually we call a sirnrm-spore. Ulothrix repro- 

 duces also by motile gametes in which may be discerned occasionally 

 a slight inequality in size suggesting the beginnings of difference 

 in sex although for the most part they appear quite alike. These 

 sexual or subsexual gametes arise from the cells of the filament in 

 much the same way as the swarm-spores do, but thej^ are more 

 numerous and smaller, and possess only two flagella (D, E). They 

 unite sidewise (F) with their tips together, thus producing what 

 looks like a swarm-spore (G), with its four flagella, but which differs 

 in having two eye-spots. A protoplast resulting from the fusion of 

 two protoplasts, whether they be alike or unlike, is termed a zygote.- 

 The zygote of Ulothrix soon absorbs its flagella (//), becomes round, 

 and secretes a cellulose wall, thus becoming a resistant zygospore 

 ready for a period of rest. The zygospore germinates by forming 

 several swarm-spores {K) each of which in turn grows into a thallus 

 as already described. 



In the sheath algae (Coleochaete) the thallus (Fig. 314), 

 is in the form of a flat disk or cushion-like mass attached 

 to some support by the lower surface. This chsk as in the 

 species figured usually consists of branching filaments which 

 elongate by repeated division of the terminal cell and branch 

 by its frequent forkings. {B,a-g). In other species the fila- 



1 Fla-gol'Ium < L. a lohip. 



- Zj-'gote < Cir. z'jgolos, yoked. 



