DIDYMOCLADON. 



[ 262 ] 



DIDYMOSPORIUM. 



of Aspidieae (Polypodiaceous Ferns), with a 

 curious elliptical indusium opening on each 

 side (figs. 175 & 176). Exotic. 



DIDYMO'CLADON, Kalfs. A genus 

 of Desmidiaceae. 



Char. Cells single, constricted at the 

 middle, end view tri- or quadrangular ; each 

 angle with two processes, one lateral and 

 in the front view nearly parallel with the 

 corresponding one of the other segment, 

 the other superior and divergent. 



The two processes distinguish this genus 

 from Staurastrum. 



D.furcigerus (PI. 14. fig. 32, front view; 

 fig. 56, end view^). 



a, end view triangular. 

 /3, end view quadrangular. 

 Length, including processes, 1-830". 

 BIBL. Ralfs, Brit. Desmid. 144. 

 DIDYMOHE'LIX, Griffith. A genus of 

 Oscillatoriaceae (Confervoid Algae), with the 

 threads consisting of pairs of microscopic, 

 interlacing, ferruginous, spiral filaments j 

 probably surrounded by gelatine. 



It seems to have no relation with Gal- 

 lionella as formerly supposed. 

 Found in ferruginous bog- water. 

 The structure of the compound filaments 

 of which this beautiful and curious organism 

 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". Filaments imbued 

 with peroxide of iron, containing no silica, 

 or not more than a trace, such as is naturally 

 invariably associated with the peroxide. 

 "With hydrosulphuret of ammonia, they be- 

 come black. Acted upon slowly with di- 

 lute muriatic acid, the colour gradually 

 vanishes, a very transparent colourless cast 

 of the original being left. If the compound 

 filaments are macerated for some time in 

 distilled water, the filaments will separate 

 (PI. 1. fig. 10 d). Under a J-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. 

 10 6, 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 coyer and of the liquid, or \vhen 

 lying edgewise. When lying flat upon the 

 slide, and the correction is perfect, they ap- 

 pear as in PI. 1. fig. 10 c. 



In the natural state, a quantity of yel- 

 lowish-brown gelatinous matter is always 



found in the water containing the filaments. 

 In the ferruginous gelatine are found some 

 fibres of a very minute Nostochaceous 

 plant, probably Anabaina subtilissima, 

 Kiitz. Didymohelix is by no means com- 

 mon, even in waters which contain a very 

 copious ferruginous deposit. It is found in 

 ditches in Dumfriesshire, 10 or 12 inches in 

 thickness. 



It may be preserved either in the dry 

 state, in chloride of calcium, or in balsam. 



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-OBJECTS. 

 BIBL. Griffith, Ann. Nat. H. 1853, xii. 

 438 ; Davies, M. M. Jn. 1877, xvii. 54. 



DIDYMO'PRIUM, Kiitz. A genus of 

 Desmidiacese. 



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 

 number of angles. 



D. Borreri (PL r l. fig. 11). Joints in- 

 flated, barrel-shaped, longer than broad : 

 side view circular ; angles bicrenate. (Sheath 

 wanting or indistinct.) 



The delicate longitudinal lines were pro- 

 prosed by Jenner as a test- object for the 

 power of the microscope; they are best seen 

 in the empty cells when dried. Breadth 

 of filament 1-1030". 



D. Gremllii (PI. 14. fig. 5 ; fig. 6, side 

 view). Joints broader than long, with a 

 thickened border at their junction ; side 

 view broadly elliptic; angles bidentate. 

 (Sheath distinct.) Breadth of filament 

 1-470". 



BIBL. Ralfs, Brit. Desmid. p. 55. 

 DIDYMOSPO'RIUM, Nees. A genus 

 of Melanconiei (Stylosporous Fungi), grow- 

 ing upon shoots of trees. The only British 

 species, D. profusum, Grev., has very mi- 

 nute, oblong, uniseptate spores, at first glued 

 together like a depressed conical nucleus, 

 beneath the epidermis, afterwards bursting 

 through and becoming free. D. elevatum } 

 Lk. = Melanconium bicolor, Nees. 



Greville's plant, however, has not truly 

 unicioptate spores, and is rather a Melanco- 

 nium, referred as a conidiiferous form by 

 Tulasne to Melanconium stilbostoma. 



