IMEDULLARY SHEATH. [ 496 ] 



MELANCONIEI. 



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- 



Fig. 457. 



Section of a four years' old shoot of the Cork oak. M, pith ; 

 1, 2, 3, 4, medullary rays of successive years ; P. C, liber layers ; 

 S, cork layers. 



Magnifled 20 diameters. 



ferently ; this causes the "silver grain" as 

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

 sections of the trunk, the ends of the me- 

 dvdlary rays usually appear as short, more 

 or less regidar, narrow streaks. 



MED'ULLAR Y 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. 4oo 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 medullaiy 

 sheath. It is absent in some Dicotyle- 

 donous stems, for example in the Oroban- 

 chacese. 



MEDU'S^E. See Acaleph^. 



MEE'SIA, Hedw.— A genus of Bartra- 

 mioid Mosses ; one species, M. uliginosa 

 ( = Bnjiim trichodes) , certainly British ; 

 another. 31. lonr/iseta, doubtful. 



MEESIA'OKE.— AtribeofBartramioid 

 Mosses, containing two genera, of which 

 there are but few British representatives. 

 See Meesia and Paludella. 



MEGALOT'ROCHA, Ehr.— A genus of 

 Rotatoria, of the family Megalotrocha^a. 



Char. Eyes two, red, sometimes disap- 

 pearing with age. 



Rotatory organ two-lobed or horse-shoe- 

 shaped ; teeth in rows. 



M. albo-flavicans, E. (PI. 44. fig. 1). 

 Colourless and unattached when young, 



yellowish and grouped in radiant clusters 

 when old; freshwater; 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. Ehr. Lifus. 396; Gosse, 

 Ann. N. H. 1851, viii. 198 ; Pritch. 

 Infus. 



MEGALOTROCH/E'A, Ehr.— a 

 family of Rotatoria. 



Char. Neither envelope nor cara- 

 pace present ; rotatory organ simple, 

 notched or sinuous at the margin. 

 Three genera : 



Eyes none Cyphonautes. 



Eyes present 



Eye one Microcodon. 



Eyes two JJegalotrocha. 



BiBL. Ehr. Infus. p. 394. 

 MEGAM'ERUS, Duges.— a genus 

 of Trombidiua (Acariua). 



Char. Palpi long, fi-ee, with a claw ; 

 mandibles forcipate; body constricted; 

 coxse^ distant ; legs ambulatoiy — femora, 

 especially _ of the fourth pair, "very large, 

 seventh joint short. 



Several species. They live in damp 

 shady places, and move rapidly. 



M. celer (PL 6. tig. 33 : a, labium ; b, 

 palp). Minute; abdomen oblong; the sides 

 narrowed posteriorly, covered with hairs, 

 and with three terminal set«; labium bifid ; 

 mandibles with a movable, elongated, pointed 

 and curved claw.. * 



PI. 6. fig. 33 c, mandible of M. roseus. 

 BiBL. Duges, Anyi. Sc. Nat. 2 ser. ii. 50 ; 

 Walck. Arachn. iii. 169 ; M 

 Ent. 115. 



Gervais, 



Lurray, 



MEGAP'ORA, Hincks, = Lepralia pt. 

 (Hincks, Pohjz. 171.) 



MEGU'RA, Buckt.— a genus of Aphidse. 

 Black, eyes red ; on Vicia septum. (Buck- 

 ton, Aphid., Bay Sue. i. 188.) 



MELAMPSO'RA, Cast— a genus of 

 Uredinei (Coniomycetous Fungi), distin- 

 guished by producing two distinct kinds of 

 spores — summer and winter spores. The 

 species are very common on the willow, 

 birch, poplar, &c., forming yellow or orange 

 spots upon the leaves. (Cooke, Handh. 

 522 ; Tulasne, Ann. Sc. N. 4. ii. 94.) 



MELANCONI EL— A provisional family 

 of Stylosporous Fungi, distinguislied from 

 SphiTcronemei by the perithecium being ob- 

 solete or altogether wanting. The spores, 

 which vary much in the different genera, 

 are ultimately protruded in the form of 



