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may be produced at any time from November to April. Zoospores 
are formed during the period of vegetative development, and aplano- 
spores and akinetes during the period of decline. Prominent among 
the winter annuals are: Conferva minor Klebs, C. bombycina Agardh, 
C. utriculosa Kiitz., Vaucheria geminata (Vauch.) DC, V. sessilis 
(Vauch) DC, Draparnaldia plumosa (Vauch.) Ag., Tetraspora lubrica 
(Roth) Ag., Stigeoclonium lubricum varians (Hazen) Collins, Gom- 
phonema angustatum Grun., G. acuminatum Ehrb., Spirogyra tenuissima 
(Hass) Kutz., S. inflata (Vauch) Kiitz. and Oedogonium rufescens Wittr. 
2. The Spring Annuals begin their vegetative period in late autumn 
or early spring, attain their maximum development and the repro- 
ductive stage during May. This is by far the largest of the seasonal 
groups. Among the common forms are: Zygnema stellinum (Miill.) 
Ag., Z. leiospermum De Bary, Z. insigne (Hass.) Kiitz., Spirogyra 
catenaeformis (Hass.) Kiitz., 5. varians (Hass.) Kiitz., S. protecta 
Wood, Mougeotia scalaris Hass., M. robusta (DeBary) Wittr., Oedo- 
gonium echinospermum A. Braun, Oe. multisporum Wood, Oe. suecicum 
Wittr., Bulbochaete crassiuscula Nordst., Debarya decussata Transeau, 
Vaucheria hamata (Vauch.) DC, Draparnaldia Ravenellii, Wolle, 
Coleochaete scutata Breb., and Herpesteiron confervicola Naeg. 
3. The Summer Annuals germinate in the spring. The greatest 
reproductive activities occur in July and August. Zoospores are most 
frequent in spring and early summer. Aplanospores develop mostly 
in August and September. Among the common summer annuals are 
Oe. praticolum Transeau, Oe. varians Wittr., Oe. vaucherii (LeCl.) 
Braun, Calothrix stagnalis Gomont, Spirogyra ellipsospora Transeau, 
5. nitida (Dillw.) Link, S. irregularis Nageli, S. setiformis (Roth) 
Kiitz., Mougeotia sphaerocarpa Wolle, and Cylindrocapsa geminella 
minor Hansgirg. 
4. The Autumn Annuals. These species begin their vegetative 
development in late spring, increase through the summer and have 
their maximum abundance in the autumn. Sexual reproduction if 
present, occurs mostly in September and October. This is a com- 
paratively small group, and includes Rivularia natans (Hedw.) Welw., 
Oedogonium capilliforme Kiitz., Oe. macrandrium Wittr., Oe. obtrun- 
catum Wittr., and Oe. crassum amplum (M. & W.) Hirn. 
5. The Perennials. This group includes species whose vegetative 
cycle may be or is continuous from year to year, in permanent streams 
and ponds. Reproduction may occur at any time in some of the 
