MEDULLARY 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- 



PC 



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 

 8, cork layers. 



Magnified 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- 

 dullary rays usually appear as short, more 

 or less regular, narrow streaks. 



MED'ULLARY 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. 455 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 Dicotyle- 

 donous stems, for example in the Oroban- 

 chaceae. 



MEDU'S^E. See ACALEPHJE. 



MEE'SIA, Hedw. A genus of Bartra- 

 mioid Mosses; one species, M. uliginosa 

 (= Bryum trichodes), certainly British; 

 another, M. longiseta, doubtful. 



MEESIA'CEvE. AtribeofBartramipid 

 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 Megalotrochaea. 



Char. Eyes two, red, sometimes disap- 

 pearing with age. 



Rotatory organ two-lobed or horse-shoe- 

 shaped ; teeth in rows. 



M. albo-favicans, 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. Infus. 396; Gosse, 

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

 In/us. 



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 Megalolrocha. 



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

 MEGAM'ERUS, Duges.-A genus 

 of Trombidina (Acarina). 



Char. Palpi long, free, with a claw ; 

 mandibles forcipate; body constricted; 

 COX89 distant ; legs ambulatory femora, 

 especially of the fourth pair, very large, 

 seventh joint short. 



Several species. They live in damp 

 shady places, and move rapidly. 



M. celer (PI. 6. 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 movable, elongated, pointed 

 and curved claw. 



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



BIBL. Duges, Ann. Sc. Nat. 2 se*r. ii. 50; 

 Gervais. Walck. Arachn. iii. 169 ; Murray. 

 EC. Ent. 115. 



MEGAP'ORA, Hincks, = Lepralia pt. 

 (Hincks, Pohjz. 171.) 



MEGU'RA, Buckt A genus of Aphidrc. 

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

 ton, Aphid., Ray Soc. 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, Handb. 

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



MELANCONI'EL A provisional family 

 of Stylosporous Fungi, distinguished from 

 Sphseronemei 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 



