PLEUROCOCCUS 87 



75. Tolypothrix or Scytonema. These tjrpes are especially interesting for 

 the peculiar method of branching, called false branching. Study the general 

 morphology of the filament with special reference to the relation of the 

 branches to the heterocysts. Find the beginnings of a branch and note that 

 it breaks through the sheath which incloses the vegetative cells. The 

 heterocysts are more or less firmly united to the sheath, while the vegeta- 

 tive cells may slip along within it. The multiplication and growth of the 

 vegetative cells between the heterocysts, as fixed points, bring pressure to 

 bear which results in the rupture of the sheath and formation of a branch. 



76. GlcEotiichia. This type should be studied chiefly for the remarkable 

 resting cells, or spores, formed next the heterocysts at the bases of the radiat- 

 ing filaments, and for the attenuation of the filaments into long hairs. 



THE GEEEK ALGiE, OR CHLOEOPHYCEiE 



77. Field work on the green algae. The green algae live under a variety of 

 conditions, with several characteristic habitats : (1) there are the growths 

 in clear pools and ponds with floating filamentous masses (pond scums), free-. 

 swimming forms (members of the Volvox family), attached filamentous or 

 expanded types {CEdogonium, Vaucheria, ChoBtophora, Coleochxte, etc.), and 

 the sediment, rich in desmids, diatoms, and many other one-celled types ; 

 (2) there are the growths in slowly running water of streams and on 

 the borders of lakes (frequently Ulothrix, Stigeoclonium, Draparnaldia, 

 Cladophora, and the stoneworts) ; (3) there are the growths just above and 

 below low-water mark on rocks along the seacoast (chiefly sea lettuces, 

 Ulothrix, and Cladophora) ; (4) there are the slimy growths on the trunks of 

 trees and stone walls (Pleurococcus and other one-celled relatives), and 

 filamentous forms on the earth {Vaucheria). 



Studies should be made of some of these habitats, collections gathered 

 and examined in the laboratory, and the principal genera identified. Notes 

 should be taken in the field describing the appearance of the algae as regards 

 size, texture, and color, and their growth habits in relation to light, depth 

 of immersion, and other factors. 



78. Pleurococcus.* * Gather pieces of green stained bark from 

 the north side of trees, or scrapings from old fences. 



A. Note the color and powdery appearance of the growth over 

 the surface and its thickness in places where the growth 

 separates as small scales. Moisten the bark and note the 

 brighter color. 



