NITBLLA TENUISSIMA. 123 



The first forking is apparently always sterile, the place 

 of the antheridium being taken by a central ray, thus pro- 

 longing the main axis. 



The outer membrane of N. tenuissima exhibits a very 

 marked and distinctive decoration seen in no other British 

 species. In the early stages of its development this decora- 

 tion is simply reticulate, but as the growth advances the 

 threads of the reticulations give early indication of uneven 

 lines as though developing minute knots at close intervals. 

 These knots soon become conspicuous and decorate the 

 reticulation-lines with delicate little beads. The patterns 

 of the reticulations are infinite in form and arrangement and 

 are never found repeated. The inner coloured membrane, 

 which adheres very closely to the outer and is not easily 

 disengaged, is very light yellow in colour, thin, and trans- 

 parent. Its decoration bears no resemblance to that of the 

 outer membrane, being obscurely granulate. 



The very restricted disti'ibution of this species is remark- 

 able. It is most plentiful in the fenlands of Cambridgeshire, 

 its headquarters apparently being in Wicken Lode, a large 

 fen drain running through Wicken Fen and emptying itself 

 into Bottisham Lode, which in its turn falls into the Cam at 

 Upware. In this lode the little plant abounds, covering the 

 whole bed of the dyke in many places with its thick mosslike 

 growth, though often scarcely discernible on account of the 

 muddy deposit which envelops it. It is found also in the 

 peat holes and ditches of the adjacent fen, but never appears 

 to have established itself in the same abundance in Bottisham 

 Lode. Indeed below the spot where the Wicken waters pass 

 into it there appears to be little or no trace of the plant. 



The occurrence of JV. tenuissima in the Norfolk fens seems 

 very limited, a few isolated plants only having been dis- 

 covered there ; its distribution in its third station, Anglesey, 

 is also much restricted. Though in Ireland it has up to the 

 present only been collected in two counties, there is every 

 reason to suppose that its distribution is more extensive. 



Ripe fruit does not appear to be common in the British 

 plants. A search through many specimens collected during 

 a series of years in Wicken Fen in early and late summer, 

 while yielding abundant oogonia and antheridia, revealed no 

 ripe oospores. The fruit of the Anglesey plant seems to be 

 equally undeveloped. On the other hand, specimens from 

 Norfolk and West Meath contained abundant oospores. 



