Io6 CLASSIFICATION OF PLANTS 



narrower middle part of the cell and the chromatophore is symmetrically 

 doubled. 



236. Order 2. — The Zygnemacece are always filamentous and the cells 

 are cylindrical. In conjugation the entire contents of the cells is involved. 



237. Order 3. — The Mesocarpacece are similar to the foregoing but 

 only a part of the cell contents is concerned in conjugation. 



238. Class 5. Chlorophyceae.— The Chlorophyceae are a 

 large group of fresh water and marine chlorophyll green algae. 

 They reproduce ascxually by the formation of pear-shaped zoo- 

 spores which have two or four flagellae. Sexual reproduction 

 usually consists in the conjugation of similar zoospores but 

 there is often a differentiation of gametes into eggs and sperms. 



239. Order i. — The Volvocales are motile throughout life. They are 

 usually single and resemble the green flagellates, but some forms adhere 

 by their gelatinous walls and form swimming colonies. The cell has a 

 single nucleus and a chromatophore. 



240. Order 2. — The Protoccocales are similar to the Volvocales but are 

 only motile in the zoospore stage. 



241. Order 3. — The Ulotrichales are usually simple or branched fila- 

 mentous forms but some marine species form flat ribbons of two layers of 

 cells. The cells are uninuclear and have usually one chloroplast. 



242. Order 4. — The Siphonocladiales are also filamentous forms, usually 

 much branched. The filaments are composed of large multinuclear cells 

 with one or more chloroplasts. 



243. Order 5. — The Siphonales consist of a branching tubular thallus 

 with few or no cross walls in the vegetative condition. The protoplasmic 

 substance is therefore continuous, with numerous nuclei and chloroplasts. 



244. Class 6. Characeae.-^The Characeae are fresh-water 

 algae of rather complicated structure and with highly differ- 

 entiated gametes and gametangia. The principal axis of the 

 thallus consists of alternately long and short tubular cells form- 

 ing nodes and internodes. A whorl of branches occurs at each 

 node and the branches resemble the main axis in structure. 

 Short branches of a second order may also occur and in the 

 axils of these are found the oogonia and antheridia. The first 



