106 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 asexually 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 



