DIDYMOCHLJ2NA. 



[ 209 ] 



DIDYMOPRIUM. 



DIDYMOCHL.ENA, 



Desv. A genus of Di- 

 plasieae ( Polypodseous 

 Ferns), with a curious 

 elliptical indusium open- 

 ing on each side (figs. 

 178 & 179). Exotic. 



Fig. 179. 



Didymochlaena sinuosa. 



Fig. 178. A sorus from above. Magn. 20 diams. 

 Fig. 179. Transverse vertical section of ditto. 



DIDYMOCLADON, Ralfs. A genus of 

 Desmidiaceae. 



Char. Cells single, constricted at the mid- 

 dle, end view tri- or quadrangular; each 

 angle with two processes, one lateral and in 

 the front view nearly parallel with the corre- 

 sponding one of the other segment, the other 

 superior and divergent. 



The two processes distinguish this genus 

 from Staurastrum. 



D. furcigerus (PI. 10. fig. 32, front view; 

 fig. 56, end view). 



a, end view triangular. 



/3, end view quadrangular. 



Length, including processes, 1-330". 



BIBL. Ralfs, Brit. Desmid. p. 144. 



DIDYMOHELIX, Griffith. A genus of 

 Oscillatoriacese (Confervoid Algae), with the 

 threads consisting of pairs of microscopic, in- 

 terlacing, flattened, ferruginous, spiral fila- 

 ments. (Probably surrounded by gelatine.) 



D. ferruginea (Gallionella ferrug., Ehr., 

 Glceotila ochracea, Kiitz.). 



Found in ferruginous bog-water. 



The structure of the compound filaments 

 of which this beautiful and curious or- 

 ganism consists, requires great care to elicit, 

 both on account of their minute size and 

 their peculiar form. The breadth of the 

 filaments is from 1-5000 to 1-30,000", the 

 average 1-10,000 to 1-20,000". The fila- 

 ments are imbued with peroxide of iron, but 

 they contain no silica, or at least not more 

 than a mere trace, such as is naturally inva- 

 riably associated with the peroxide. When 

 treated with hydrosulphuret of ammonia, 

 they become black. When acted upon slowly 

 with dilute muriatic acid, the colour gradu- 

 ally vanishes, a very transparent colourless 

 cast of the original being left. If the com- 

 pound filaments be macerated for some time 



in distilled water, the filaments will separate 

 (PL 1. fig. lOd). When examined with a 

 -inch object-glass, the filaments present 

 the appearance represented in PI. 1. fig. 10 a. 

 When a higher power is used, they appear 

 as in fig. 106, which represents them as seen 

 when too much liquid is contained between 

 the slide and the cover, or when the proper 

 correction is not made for the thickness of 

 the glass cover and of the liquid, or when 

 they are lying edgewise. When lying flat 

 upon the slide, and the correction is perfect, 

 they appear as in PI. 1. fig. lOc. 



Ehrenberg considered them as represent- 

 ing Polygastric Infusoria, Kiitzing as con- 

 sisting of cells. These views, however, are 

 based upon inaccurate observation, or the 

 substitution of analogy for observation, and 

 need no consideration. 



In the natural state, a quantity of yellow- 

 ish-brown gelatinous matter is always found 

 in the water containing the filaments. Eh- 

 renberg supposed that they are formed in or 

 from this. We have always found in this 

 ferruginous gelatine some fibres of a very 

 minute Nostochaceous plant (probably Ana- 

 baina subtilissima, Kiitz.). Perhaps the pre- 

 sence of the Nostoc is accidental, because a 

 proper soil is present in the ferruginous 

 water for its growth. 



The Didymohelix is by no means common, 

 even in waters which contain a very copious 

 ferruginous deposit. 



Didymohelix may be preserved either in 

 the dry state, in chloride of calcium, or in 

 balsam; perhaps the chloride is the best. 

 Balsam renders it very transparent. 



We have enumerated this as a test-object 

 for the general excellence of a high-power 

 object-glass ; also of the observer's manage- 

 ment of the microscope. See TEST-OB- 

 JECTS. 



BIBL. Ehr. In/us.', Kiitzing, Sp. Alg. p. 

 363 ; Ralfs, Ann. Nat. Hist. 1843. xii. p. 351; 

 Grif. Ann. Nat. Hist. 1853. xii. p. 438. 



DIDYMOPRIUM, Kiitz. A genus of 

 Desmidiaceae. 



Char. Cells with a bidentate or bicrenate 

 process on each side, united into an elon- 

 gated, fragile, cylindrical, and regularly 

 twisted filament. (Sheath either present, 

 wanting or indistinct.) 



Differs from Desmidium in having only 

 two processes, and not being angular, and in 

 the number of rays of the endochrome in 

 the side view not depending upon the num- 

 ber of angles. 



D. Borreri (PI. 1 . fig. 1 1 ) . Joints inflated, 



