198 BRITISH DESMIDIACE^. 



genus is distinguished from Dae id in in by the entire 

 absence of plications at the base of the semicells, by the 

 commonly tuberculated or peripherally plicated apices,, and 

 by the nature of the chloroplasts. 



Few species of the genus are destitute of tubercles round 

 (he apex, and in some, such as PL Sceptrum, i~he tubercles 

 are replaced by sharp teeth. The genus is poorly represented 

 in the J>ritish Islands, being most abundantly met with in 

 the tropics. Many of the tropical species are very prettily 

 marked, and some of them are attached by their tuberculated 

 apices so as to form long filamentous colonies. 



The nature of the parietal chloroplasts is well illustrated 1>\ 

 the drawing of the semicell of PL conniat/mi on PL XXVIII 

 (iig. 4). The central cavity of the cylindrical semicells is 

 occupied by a number of large fluid vacuoles, the terminal 

 one frequently containing a mass of small moving granules. 

 Under abnormal conditions these granules make their appear- 

 ance in all the vacuoles of the cell. 



At the isthmus or point of junction of the old and new 

 semicells is a thickening of the cell-wall, termed the xiit/tn'. 

 This projects evenly as a circular rim all round the middle of 

 the cell, and as a rule the larger the species the more promi- 

 nent the suture. It is of no use as a specific character. 



There are nine British species,* which are best arranged as 

 follows : 



SECTION A. Cells cylindrical or slightly attenuated ; eud view 

 circular. 



* Cell-wall smooth, punctate, or granulate. 



f Apices furnished with a ring of tubercles. 



1. PI. coronat/uti. 



2. PI. eugen&um. 



3. PI. truncatwm. 



4. PL Ehrenlcnjii. 



5. PL tridentulnni. 

 ft Apices without tubercles. 



G. PL Tralccula. 



7. PL maximum. 



** Cell-wall papillate. 



8. PL Hufchinsonii. 



SECTION B. Cells furnished with rings of nodules; end view 



s i a uate- stellate . 



9. PL nodosum. 



* We regard Bailey's " Docidium hirsutum," which is mentioned in Cooke's 

 lirilish Desmids (p. 17, t. 7, f. 5) and doubtfully recorded by Eoy from 



Si-.p| l.-iinl, :is :i defective.' representation < 'f a species of <in<iti.i'~ifj<m. 



