ACHNANTHIDIUM. [ 



some has a nodule at each end. The valves 

 being much compressed, the transverse rows 

 of dots appear also in the front view. The 

 hoops exhibit faint longitudinal and some- 

 times transverse striae. 



Achnanthes resembles Striatella in its 

 stalked flag-like filaments, but may be known 

 from it by the absence of internal siliceous 

 plates. 



Mr. Ralfs distinguishes five species thus : 



fFluviatile, striae (wanting? or) indi- 



J stinct 2 



*'] Marine or submarine; lateral striae 



L evident 3 



rStipes much longer than the frustule; 



J lateral surfaces subacute exilis*. 



] Stipes not longer than the frustule ; 



L lateral surfaces obtuse minutissima. 



f Stipes longer than the frustule longipes^. 



\ Stipes shorter than the frustule 4 



f Lateral view lanceolate, with acute 

 4.4 ends brevipes. 



[ Lateral view elliptic, with obtuse ends subsessilis. 



* PL 12. f. 4. t K. 12. f. 1. 



Kiitzing enumerates 14 species of Ach- 

 nanthes, and 4 of Achnanthidium', Achn. 

 microcephalum, pi. 12. f. 5, Achn. delicatu- 

 lum, pi. 12. f. 6, Achn. Jlexellum (Cocconeis 

 Thwaitesii, Smith), and Achn. lanceolatum. 



ACHNANTHIDIUM. See ACHNAN- 

 THES. 



ACHORION, Link and Remak. The 

 generic name applied to one of the vegetables 

 occurring in Favus, and characteristic of 

 that disease of the skin (also called Porrigo 

 or Tinea favosa) . The structure of the plant, 

 Achorion SchoBnleinii, bears much resem- 

 blance to that of the genus Torula, but it 

 occurs in definitely bounded patches having 

 a special arrangement of the microscopic ele- 

 ments of which it is constituted. The fol- 

 lowing is the botanical character given by 

 Link and Remak. 



Achorion. Orbicular, yellow, coriaceous, 

 immersed in the human skin, especially of 

 the head. 



Mycelium soft, pellucid, floccose, filaments 

 very slender, not jointed, very much branched, 

 mostly fixed in a granular stroma. Recep- 

 tacle formed of thicker filaments composed 

 of elongated cells, somewhat branched, di- 

 stinctly articulated, joints unequal, irregular, 

 terminating in a sporidium. Sporidia round, 

 oval or irregular, germinating at one or seve- 

 ral points. Allied to Oidium. 



Achorion Schcenleinii, Remak. Character 

 the same as the genus. 



Ch. Robin gives a very full history of this 

 plant, but it will suffice to abstract the prin- 

 cipal points touching on the microscopic 



] ACHORION. 



structure, previously to presenting some re- 

 marks tending to alter the opinion commonly 

 entertained as to the nature of the so-called 

 Achorion. The plant is found upon the 

 human skin, either in the hair-follicles or in 

 depressions of the surface. With regard 

 to the former situation, it appears to be a 

 secondary seat, as it were, since only the 

 " spores" or moniliform filaments composed 

 of rows of "spores" occur therein, adhering 

 firmly to the hair and forming a kind of 

 sheath around it. When it occurs upon the 

 ordinary surface of the skin, it forms a little 

 mass, like a little cup, the favus, which is at 

 first developed beneath the epidermis, and 

 laid bare afterwards by desquamation. The 

 favus is somewhat hemispherical in general 

 form, and varies from 1-25 to 3-5ths of an 

 inch in diameter, its depth or thickness being 

 from 1 -25th to l-6th or l-5th of an inch. The 

 upper, free side is concave, the lower convex, 

 the colour is pale sulphur yellow, sometimes 

 a little browned by the presence of foreign 

 bodies. The cup-like depression existing at 

 first becomes filled up with advancing growth, 

 and when the /aw have acquired a consider- 

 able size, concentric lines are perceived upon 

 the upper surface. The circumference of the 

 free upper surface adheres to the epidermis, 

 and the mass is generally traversed by one 

 or two hairs, passing completely through it 

 from below. When a vertical section is made 

 of a favus dissected out of its seat, it is found 

 to be composed of the following elements. 

 The periphery consists of a granular crust, 

 about 1-150 of an inch in diameter, the 

 stroma, apparently a hardened exudation 

 from the surrounding parts ; this is lined by 

 the mycelium passing in from it, composed 

 of flexuose, branched, inarticulate filaments, 

 uniform in thickness (at most 1-8000 of an 

 inch). Next the mycelium, proceeding in- 

 wards, come the receptacles or 'sporophores,' 

 consisting of tubes analogous to those of the 

 mycelium, less flexuose, the fertile being 

 more or less straight, terminating in strings 

 of spores. These receptacles are from 1-500 

 to 1-125 of an inch long, and from 1-25000 to 

 1-5000 of an inch in diameter. The spores 

 are round or oval, the smallest 1-8000 to 

 1-6000 inch, the largest 1-5000 to 1-4000 

 inch in diameter, the oval are as much as 

 1-3500 to 1-2500 in length; the spherical 

 sometimes 1-3500 in diameter. Their mem- 

 brane is well-defined, water and acetic acid 

 do not affect them. Their contents are 

 strongly refractive and homogeneous, ex- 

 cepting from the presence of a veiy fine 



