STRUCTURE AND DEVELOPMENT. 69 



ridges become broader at the apex of the oospore and 

 form a sort of crest. 



The outer coloured membrane in almost 

 ^ f°t'>i ^°^ ^^^ species exhibits a decorated surface. 



membrane ^^ ^^^^Y developed membrane the decora- 

 tion spreads on to the ridges and is also 

 found on the flanges when the latter are produced 

 (Plate IV, f. 2). 



To Dr. Nordstedt we are indebted for pioneer 

 work in the study and classification of these mem- 

 brane decorations and his monograph on the subject 

 (Part IV of his ' De Algis et Characeis ') has proved 

 invaluable in this branch of research. The character of 

 the decoration of the membrane is apparently constant 

 in the same species, and therefore often affords valuable 

 aid to the determination of species. 



Of the British Charophytes only four — Nitella 

 ■opaca, Nitella fiexilis, Tolypella prolifera, and Tolypella 

 nidifica — are without a decorated membrane. For the 

 rest the decoration falls into three classes— (a) tuber- 

 ■culate, (b) reticulate, (c) granulate. Under the first 

 head N. capillaris stands alone. The tubercles of 

 the membrane of this species are strongly marked, 

 standing out detached and prominent (Plate IV, f. 1). 

 Under the second head fall N. tranducens, N. mucro- 

 nata and N. teimissiina (Plate IV, f. 10; V, figs. 1, 

 3, 4) ; the distinctive characters of the reticulation 

 •of these three species are obvious and well-marked. 

 The remaining 24 British species fall under the third 

 •class, viz. granular membranes. But the granules of the 

 different species vary very widely in size, disposition, 

 ■and character. The granulation in its earlier stages 

 is found not infrequently arranged in linear form 

 parallel with the ridges. As the granule's develop 



