88 BRITISH CHAROPHYTA. 



The British species of Charophyta are for the most 

 part fairly distinct, but, except in the case of a few 

 which are particularly compact and well marked, the 

 various organs, especially those of the vegetative 

 system, are liable to considerable variation, and 

 aberrations from what is apparently the normal form 

 of an organ in a particular species are common. In 

 consequence of this we have been obliged to make 

 more frequent use of the word " usually ' in the dia- 

 gnoses than we should wish. In the case of the more 

 protean species, were the descriptions framed so as to 

 include every variation to be met with in point of 

 number, size and form of the various organs, there 

 would be little left in the shape of characters to 

 distinguish them. 



The measurement of the internodes is taken where 

 the stem has lengthened out and does not apply to 

 those between the close -growing upper whorls. 



The extent to which the spiral-cells of the oogonia 

 and the ridges on the oospore are twisted, in the 

 several species, is denoted by stating the number of 

 convolutions and ridges visible from one side (the 

 broader dimension in the case of Nitellds), as being 

 simpler than attempting to state in fractions the 

 precise extent traversed by each spiral-cell. 



In the matter of nomenclature, the rule of strict 

 priority has so far as possible been followed. In doing 

 this we have been obliged, very reluctantly, in a few 

 cases to supersede specific names generally in use, in 

 favour of those published earlier. The temporary 

 inconvenience of such a course is fully recognized, but, 

 if the ' International Rules ' are to be accepted, the 

 alterations must be made sooner or later, and it seems 

 to us, therefore, " the sooner, the better." 



That wonderfully acute and clear-sighted botanist, 

 Alexander Braun, was the first who can be said to 

 have acquired any real grasp of the distinctive char- 

 acters of the Charophyta from a systematic point of 

 view, and, prior to the commencement of his publica- 



