254 BOTANY. 



{d) The genus VaucJieria may be taken as the type of a group, the 

 Vaucheriacece, but whether it is entitled to rank as an order instead 

 of a family cannot be decided in this place. Allied to Vaucheria are 

 Vcmlerpa, Halimeda, etc., but their exact position is as yet problematical. 



(«) Thirteen species of Vaucheria occur in the fresh waters of the 

 United States, one of the most common being V. sessilis, which 

 occurs everywhere in brooks and springs. 



(/) Oaulerpites cactMes is the oldest known fossil species of this 

 elass. It occurs in the Silurian : other species have been detected in 

 the Devonian and Tertiary. Caulerpa extends from the Tertiary to 

 the present. 



837.— Order Saprolegniacese. The jjlants of this order 

 are saprophytes or parasites, more frequently the latter ; they 

 are colorless, and generally are to be found in the water or in 

 connection with moist tissues. The plant-body is greatly 

 elongated and branched, and all its vegetative portion is 

 continuous — i.e., unicellular ; the reproductive portions only 

 are separated from the rest of the plant-body by partitions. 



338. — The reproduction is very much the same as in 

 Vaucheria, and, as in that genus, is of two kinds — asexual 

 and sexual. The asexual reproduction may be briefly de- 

 scribed as follows : the protoplasm in the end of a branch 

 becomes somewhat condensed, a septum forms, cutting off 

 this portion from the remainder of the filament, and the 

 whole of its contents becomes, converted by internal cell- 

 division into zoospores provided with one or two cilia 

 (Fig. 171, 1). These soon escape from a fissure in the wall 

 and are aative for a few minutes (3-4), after which they 

 come to rest and their cilia disappear (3 and 3, Fig. 171). 

 In one or two hours they germinate by sending out a filament 

 (4, Fig. 171), from which a new plant is quickly produced.* 



339. — The sexual organs bear a close resemblance to those 

 of Vaucheria. The oogonia are spherical, or nearly so (in 

 most of the species), and contain from two to many oospheres,. 

 which are fertilized by means of antheridia, which usually 

 develop as lateral branches just below the oogonia. In 



* The student is referred to an article, "Observations on Several 

 Forms of Saprolegniese," byF. B. Hine, in American Quarterly Micro- 

 scopical Journal, 1878, p. 18, from which some of the above facts are 

 taken, and the accompanying figures adapted. 



