STRUCTURE AND DEVELOPMENT. 63 



characteristics in their vegetative parts. The anthe- 

 ridia of dioecious species are usually much larger than 

 those of the most nearly allied monoecious species. 



In most of the monoecious species the antheridia 

 and oogonia are produced at the same nodes of the 

 branchlets. In a few extra-British species they are 

 produced on the same branchlets, but at different 

 free nodes, while in others the oogonia are mostly 

 produced at the basal nodes and the antheridia only 

 at the free nodes. "We have seen an instance of the 

 two organs being developed on separate branchlets 

 of the same whorl. These exceptions to the general 

 rule may be regarded as deviations in the direction 

 of dioecism. 



The Charophytes are usually protandrous, sometimes 

 in a marked degree. So much is this the case in a 



o 



variety of Nitella flexilis, that at an early stage the 

 plant appears entirely male, and at a late stage 

 entirely female, and the antheridia produced in the 

 early stage are as large as those of the dioecious 

 species belonging to the same section. 



In some instances the distinction between two 

 species appears to consist almost entirely in their 

 monoecious or dioecious character. For example, 

 apart from this distinction there is a close similarity 

 between Cliara canescens and the American C. hirsuta; 

 between the French Nitella ornithopoda and the 

 Portuguese N. Dixonii; and most markedly perhaps 

 between our two common British species -V. flexilis 

 and N. opaca, which are almost impossible to dis- 

 criminate when not in fruit. 



One species, Char a canescens, which is dioecious 

 presents the curious phenomenon of parthenogenesis. 

 The female plant, found in a few localities in the 



