[84 



PLANT BIOLOGY 



and lakes, and many are found on moist ground and on dripping 

 rocks. Aside from these, all the plants commonly known as seaweeds 

 belong to this category ; these latter are inhabitants of salt water. 

 The simplest forms of algae consist of a single spherical cell, 

 which multiplies by repeated division or fission. Many of the 

 forms found in fresh water are filamentous, i.e. the plant body 

 consists of long threads, either simple or branched. Such a plant 

 body is termed a thallus. This term applies to the vegetative 

 body of all plants that are not differentiated into stem and leaves. 

 Such plants are known as thallophytes (p. 181). All algae contain 

 chlorophyll, and are able to assimilate carbon dioxid from the air. 

 This distinguishes them from the fungi. 



Nostoc. — On wet rocks and damp soil dark, semitransparent 

 irregular or spherical gelatinous masses about the size of a pea are 

 often found. These consist of a colony of contorted filamentous 

 algae embedded in the jelly-like mass. The chain of cells in the 

 filament is necklace-like. Each cell is homogeneous, without 

 apparent nucleus, and blue-green in color, except one cell which 

 is larger and clearer than the rest. The plant therefore belongs 

 to the group of blue-green alga. The jelly probably serves to 

 maintain a more even moisture and to provide mechanical protec- 

 tion. Multiplication is wholly by the breaking 

 up of the threads. Occasionally certain cells 

 of the filament thicken to become resting- 

 spores, but no other spore formation occurs. 



Oscillatoria. — The blue-green coatings 

 found on damp soil and in water frequently 

 show under the microscope the presence of 

 filamentous algae composed of many short 



Fig. 



264. — Filament of Osciixatoria, showing one 

 dead cell where the strand will break. 



homogeneous cells (Fig. 264). If watched 

 closely, some filaments will be seen to wave 

 back and forth slowly, showing a peculiar power 

 of movement characteristic of this plant. 

 Multiplication is by the breaking up of the 

 threads. There is no true spore formation. 



Spirogyra. — One of the most common forms 

 of the green algae is spirogyra (Fig. 265). This 



Fig. 265. — Strand 

 of Spirogyra, 

 showing the chlo- 

 rophyll bands. 

 There is a nu- 

 cleus at a. How 

 many cells, or 

 parts of cells, are 

 shown in this fig- 

 ure ? 



