DESMID FLORA OP A TRIASSIC DISTRICT. 143 



frequently paved witli pebbles redeposited from the beds above. 

 The depth of water in such bogs is seldom more than an inch 

 or two, except where small depressions occur. Such bogs often 

 cover extensive patches on the sloping sides of the common, 

 creeping down to a small stream that flows through the bottom 

 and discharges into the River Otter. These shallow bogs are 

 particularly well aerated and have a rich Algal flora irrespec- 

 tive of the Desmidiaceae. One of the principal features is the 

 prolific growth of Stigonema ocellatum Dillw., which occurs in 

 extensive mats spreading over the surface of the bog. Associated 

 with this is a rich growth of Protococcoidae and Conjugatae, 

 and the exuded mucilage of the various species accumulates 

 to such an extent that large masses of it may be lifted out. 

 In a long dry spell of weather in the early summer of 1916 one 

 of these bogs presented an extraordinary sight, being largely 

 a huge collection of mucilage. In addition to the Algal flora 

 these bogs possess a hydrophytic Moss flora of considerable rich- 

 ness and variety, and a curious feature of this Bryophytic flora 

 is an endeavour to adapt itself to the shallow-water conditions. 

 Hypnum scorpioides L. is particularly noticeable for this adapta- 

 tion, though Sphagnum suhsecundum var. contortum Schp., and 

 Sphagnum suhsecundum var. turgidum CM., also exhibit it. 

 The growth takes place horizontally along the floor of the bog, 

 so that the plant is always covered by the shallow depth of water ; 

 moreover they, in common with the dicotylous macrophytes 

 growing there, are abundantly covered with the mucus excreted 

 by the algal growth. These bogs are densely populated with 

 Desmidiaceae, as the following summary from my field-book 

 will show : " Aylesbeare Common, north bog, April 29th, 1917. 

 Closterium, 22 ; Euastrum, 14 ; Micrasterias, 3 ; Cosmarium, 43 ; 

 Staurastrum, 25 ; Penium, 12 ; Various genera, 21." 



The bogs in the southern portion of Woodbury Common are 

 different, inclining rather to High Moor formation, and the 

 Desmid flora has a somewhat different facies by the inclusion 

 of species more often found in bogs at a greater altitude. 

 Xanthidium antilopaeum, X. Smithii, X. variable, Arthrodesmus 

 convergens, A. octocornis, Staurastrum brachiatum, Desmidium 

 Swartzii, D. cylindricum, Arthrodesmus trispinatus, A. suhulatus 

 var. subaequalis, and many others occur in the southern bogs 

 that do not appear in the bogs in the northern portion. The 



