ROOKS. 



[ 666 ] 



ROOTS. 



through uot risldnp: tlieir loss or diminutiou 



bv those 



finisliing 



strokes which 



of size 



render a, preparation useful or -worthless. 

 The best sections I have yet seen are those 

 prepared by Ciittel, of New Compton Street, 

 Soho. Good rock-sections may also be pro- 

 cured on the continent, notably from Yoigt 

 and Hochgesang of Vienna, Marchand 

 of Paris, and Fuess of Berlin. Machines 

 for the preparation of roelc-sections are 

 made by Cotton and Johnson of Grafton 

 Street, Soho, Fuess of BerHn, and others. 

 Mucli may, however, be done simply with a 

 flat cast-iron plate and emery, while a file and 

 hone will suffice for the preparation of some 

 of the softer rocks. Chapters on this 

 branch of microscopical work wiU be found 

 in ' The Study of Rocks,' and in Beale's 

 * How &c.' Microscopes suitable for this 

 branch of study, are made by Watson, of 

 4 Pall Mall, and Swift, of University Street, 

 Tottenham Court Road. 



The student of petrology should, how- 

 ever, study the great features presented by 

 rock-masses, and learn to recognize the 

 large crystals, with which Nature and the 

 mineral-dealer can provide him, before he 

 commits himself to microscopic investi- 

 gation. 



BiBL. Zirkel, Mtkr. Beschaff. d. Mine- 

 ralien und Gesteine, 1873 ; id. Mikr. Zitsam- 

 mensetzwng d. BascdUjesteine, 1870 ; id. 

 Microscopical Petrography, U. S. Oeoloy. 

 Explor. ofthe¥)th pnrollel, 1876; Vogel- 

 sang, Die KrystaUiten, 1875 ; Rosenbusch, 

 Mikrosliop. I'hysiograpliie, 1877 (coloured 

 plates), and the Bihl. ; Mohl, Basalte mid 

 Phonolithe Sdchsens, 1873; Poussin and 

 Renard, Caract. mineral. S^'c. des roches Plu- 

 tonnienyics de la Belyique ct de TArdenne 

 Frangaise, 187G ; Boiicky, Petrograph. 

 Stud. Basaltgesteinen Bohmens, 1874 ; id. 

 PJionolifJigesteine Bohmens, 1873; id. Me- 

 laphyrgest. 1870; id. Elemeide c/icin.-mi- 

 krosk. Mineral- und Gesteinanalyse, 1877 ; 

 (the last 4 works are in the Archiv d. Na- 

 turtvissenschaftl. Landosdtirchforschung v. 

 Bohinrn); Credner, Pefrograj>Jii-tc/ie Geologic 

 {Element e d. Geologic., 1870) ; Lasaulx, 

 Pctrographie, 1875 ; Geikie, Carboniferous 

 Volcanic Rocks of the Basin of the Firth of 

 Forth, Tr. R. Soc. Ed. 1879 ; FouqutI an'd 

 Levy, Mineralogie Micrographique, Roches 

 eruptives Fram^., coloured plates and Bibl., 

 1879; Sorby, Anniversary Addresses to Geol. 

 Soc. Land., 1878-70; Rutlev, The Study 

 of Rocks, 1879; Geikie, Text-Book, 1882. 



Numerous papers on Microscopic Mine- 



ralogy and Petrology, by Phillips, Sorby, 

 Allport, Bonney, Ward," Davis, Daintree, 

 and others, will be found in the Qu, Jn. 

 Geol. Soc, others in the Jn. Mic. Soc, Tr. 

 Roy. Soc. Ed., the R. Irish Academy, the 

 Roy. Irish Geol. Soc, Xeues Jahrh. f. 

 Mineral., Zeitsch. d. deutsch. geol. Gesellsch., 

 Bull. d. I. Soc Mineral, d. France, &c. 



RCESTE'LIA, Rebent.— A genus of Ure- 

 dinei (Coniumycetous Fungi), closely re- 

 lated to iEciDiuji, and presenting sfmilar 

 spermogonia and perithecia ; the chains of 

 spores of the Rastelice, however, present a 

 peculiarity, — having a sterile joint, forming 

 an isthmus of variable length, between 

 each spore : the peridium bursts irregularly ; 

 or (in R. cancellata) the teeth cohere more 

 or less for a time, so as to form a kind of 

 lattice. Tliis genus includes ^cidium cor- 

 nutmn, laceratum, and cancellatum of older 

 authors, growing respectively on the leaves 

 of the moimtain-ash, hawthorn, and pear. 

 See y^EciDiuM and Uuedikei. 



ROOTS. — The structure of roots presents 

 important modifications ; but these are less 

 striking than those in stems. In all cases 

 they have a fibro-vascular axis enveloped in 

 a more or less thick cortical parenchyma, 

 covered when yoimg by a delicate epidermis 

 devoid of stomata (epiblema), when old by 

 an epidermal tissue of corky nature. 



The roots of the vascidar Cryptogamia 

 are all adventitious ; and their structure 

 consists merely gf a central fibro-vascuLir 

 axis, surrounded by a cellular cortex and an 

 epidermis provided with root-hairs of a 

 yellowish colom'. 



Dicotyledons produce an axial root, which 

 is a direct prolongation of the stem down- 

 wards ; and both this and the adventitious 

 roots frequently developed on the stem have 

 the peculiar imlimited fibro-vascular struc- 

 ture found in the stems of this class, and 

 may become woody and increase by annual 

 layers lilce the ascending axis. The axial 

 root of Dicotyledons, being a direct con- 

 tinuation of the stem, displays a circular 

 group of fibro-vascular bundles as in the 

 ascending axis : but these mostly converge 

 at the point of junction of .stem and root 

 (collar), so tliat tlie central axis of the pa- 

 renchyma, the pith, is lisualty absent, the 

 medullary rays remaining as in the stem. 

 Externally, uirain, there is a difference, since 

 the liber-bundles vanish and the cambium- 

 region passes at once into the cortical pa- 

 renchyma, ' here colourless and succulent, 

 and tliis is clothed by a less prominent peri- 



