MEDULLARY SHEATH. [ 420 ] 



MELANCONIEI. 



the degree of development differ much in 

 different cases. In radial sections of Dico- 

 tyledonous wood they often appear distinctly 

 to the naked eye, from the direction of their 

 cells being different from that of the woody 

 fibre, and therefore reflecting light dif- 

 ferently ; this causes the " silver grain " as 

 it is called of oak-panels, &c. ; in tangental 

 sections of the trunk, the ends of the me- 

 dullary rays usually appear as short, more 

 or less regular narrow streaks. 



MEDULLARY SHEATH. The earliest 

 layer of fibro-vascular tissue developed in a 

 Dicotyledonous stem, consists ordinarily of 

 spiral vessels, these forming the foundation 

 of the wood-bundles (fig. 459 T). As the 

 latter stand in a circle round the pith, their 

 internal vascular layers of course form col- 

 lectively a continuous cylindrical envelope 

 to the pith ; this is called the medullary 

 sheath. It is absent in some Dicotyledonous 

 stems, for example in the Orobanchaceae. 



MEESIA,Hedw. A genus of Bartramioid 

 Mosses; one species, M.uliginosa(=Bryum 

 trichodes), certainly British; another, M. 

 longiseta, doubtful. 



MEESIACE^E. A tribe of Bartrami- 

 oid Mosses, containing two genera, of which 

 there are but few British representatives : 



I. MEESIA. Calyptra dimidiate. Peri- 

 stome double. External, of sixteen very 

 short, broad, obtuse teeth, with a median 

 line or fissile. Internal : a short membrane 

 produced into sixteen cartilaginous teeth 

 with a median line, or split and perforated ; 

 teeth often coherent with interposed, more 

 or less perfect, mostly irregularly coherent 

 appendiculiform cilia. Areolation of the 

 leaf of one character. 



II. PALUDELLA. Calyptra dimidiate. 

 Peristome double, both of sixteen teeth, 

 resembling that of Bryum, but the inner 

 without cilia. Inflorescence dioecious. Areo- 

 lation of the leaves dimorphous, lax and 

 dense. 



MEGALOTROCHA, Ehr. A genus of 

 Rotatoria, of the family Megalotrochaea. 



Char. Eyes two, red, sometimes disap- 

 pearing with age. 



Rotatory organ two-lobed or horse-shoe 

 shaped ; teeth in rows. 



M. albo-flavicans, E. (PL 35. fig. 1). Co- 

 lourless and unattached when young, yellow- 

 ish and grouped in radiant clusters when old ; 

 aquatic; length of individuals 1-36"; of the 

 clusters 1-6". 



The ova remain some time attached to the 

 parent by a cord. 



M. velata, Gosse. 



BIBL. Ehrenberg, In/us, p. 396 ; Gosse 



Ann. Nat. Hist. 1851. viii. p. 198. 



MEGALO 

 of Rotatoria. 



MEGALOTROCHAEA 



i. p. iy. 

 , Ehr. 



A family 



Char. Neither envelope nor carapace 

 present ; rotatory organ simple, notched or 

 sinuous at the margin. 



Three genera : 



Eyes none 1 . Cyphonautes. 



Eyes present. 



Eye one 2. Microcodon. 



Eyes two 3. Megalotrocha. 



BIBL. Ehrenberg, In/us, p. 394. 



MEGAMERUS, Duges. A genus of 

 Arachnida, of the order Acarina, and family 

 Trombidina. 



Char. Palpi with a claw, long, free ; body 

 constricted ; coxae distant ; legs ambulatory, 

 femora, especially of the fourth pair, very 

 large, seventh joint short; larvae hexapod, 

 resembling the adults. 



Mandibles forcipate. 



Eight species. They live in damp, shady 

 places, and move rapidly. 



T. celer (PI. 2. fig. 33 : a, labium ; b, 

 palp). Minute; abdomen oblong; the sides 

 narrowed posteriorly, covered with hairs and 

 with three terminal setae ; labium bifid ; 

 mandibles with a moveable, elongated, 

 pointed and curved claw. 



PI. 2. fig. 33 c, mandible of M. roseus. 



BIBL. Duges, Ann. d. Sc. nat. 2 ser. ii. 

 p. 50; Gervais, Walckenaer's Arachn. iii. 169. 



MELAMPSORA. See UREDO. 



MELANCONIEI. A family of Conio- 

 mycetous Fungi growing beneath the epi- 

 dermis of leaves and bark. They are at first 

 little tubercles on the surface of a white 

 mycelium, without an orifice, subsequently 

 they become hollow and fleshy, and the 

 interior becomes coated with filaments, each 

 terminating in a spore. The tubercles mean- 

 time enlarge, raise up the epidermis, and 

 appear there in groups with irregular orifices 

 opening outwards. The conceptacles are 

 closely crowded, and form blackish patches 

 on the leaves or bark, and when mature the 

 spores are expelled, mixed with filaments in 

 a gelatinous condition, and in the form of 

 threads or ribands. The spores differ in shape. 



The genera present forms which appear to 

 belong in common to Ascomycetous genera, 

 for example Cytispora is apparently a form 

 of Sphceria, &c. (See ASCOMYCETES, Co- 



NIOMYCETES.) 



Synopsis of British Genera. 

 I. MELANCONIUM. Conceptacle mem- 



