54 



PLANT MORPHOLOGY 



radiating starch grains. As in Spirogyra, sexual reproduction takes place 

 by the passage of isogametes, each representing an entire vegetative pro- 

 toplast, through conjugating tubes and their fusion in the cells of one of 

 the filaments. There is also the same degeneration of three of the haploid 

 nuclei derived from the nucleus of the zygote. 



A B 



Fig. 42. Single cells of Zygiiema, X750. A, vegetative cell, showing central nucleus and 

 two spherical chloroplasts, each with a pyrenoid surrounded by radiating starch grains; B. 

 young zygote with four chloroplasts and the two gametic nuclei not yet fused. 



Summary. The Conjugales are an aberrant order of green algae show- 

 ing no close relationship to any of the other orders. The plant body may 

 be either unicellular or multicellular, in the latter case consisting of a 

 simple unbranched filament. The cells are uninucleate and have one or 

 more peculiar chloroplasts. The distinguishing feature of the order is the 

 absence of all ciliated cells in the life history. No zoospores are produced, 

 but aplanospores may occur. Sexual reproduction is accomplished by the 

 conjugation of two noncihated isogametes, each derived from the entire 

 protoplast of a vegetative cell. These either escape and fuse, unite in a 

 conjugating tube, or pass through a conjugating tube and fuse in one of 

 the cells. 



6. Siphonocladiales 



This is a group whose members are often distributed among other 

 orders, although its characters are rather well defined. They are repre- 

 sented in both fresh and salt water, but most of them are marine, being 

 found principally in tropical and subtropical seas. Many of the marine 

 forms are incrusted with lime. Representatives of the group have been 

 found as fossils as far back as the Ordovician. The Siphonocladiales 

 include about 37 genera and 450 species, the best-known genera being 

 Cladophora, Sphaeroplea, and Acetabularia. 



Cladophora. Cladophora is a genus of about 150 species, world-wide in 

 distribution. It is found in great abundance in streams, ponds, and lakes, 

 usually attached to stones and piers. Some of its species are marine. 

 The vegetative body is filamentous and much branched, its cells being 

 elongated and cylindrical (Fig. 43A). A branch originates as an out- 

 growth from the upper end of a cell lying near the end of a filament. Each 

 cell is a coenocyte, containing many nuclei. The cytoplasm usually sur- 



