SILK. 



[ 694 ] 



SIPIIONACE^E. 



fraction-spectra, whence they were con- 

 sidered to represent and explain the forma- 

 tion of the siliceous structure of the valves, 

 and have been called Schultze's iilms. The 

 coarser ones have no resemblance whatever 

 to the coarser markings of the Diatomacese. 

 Their relation to the valves of the Diato- 

 maceae has been referred to under DIATO 

 MACE^E. 



Silica has but little action upon pola- 

 rized light; in fact, Schultze states that the 

 apparent depolarization is due to refraction. 

 The selenite-plate will, however, render it 

 more evident in many instances in which 

 it is feeble. 



BIBL. Works on Chemistry; and Schultze, 

 Verh. Naturh. Vereins. preuss. Rheinlande, 

 xx. 1, Qu. Mic. Jn. 1863, iii. 



SILK. This valuable substance is se- 

 creted in Insects, especially the silkworm, 

 by two glandular organs, described under 

 INSECTS, Spinning Organs. 



The fibres of which it is composed are 

 cylindrical or somewhat flattened, solid, 

 tolerably highly refractive, and free from 

 structural markings of any kind. 



Chemically, silk consists of a proper silk- 

 cylinder, consisting of fibroine and forming 

 the principal part of the fibres, surrounded 

 by a coat of albumen, upon which is a layer 

 of gelatine. The fibres also contain a small 

 quantity of fat and colouring-matter. 



Fibres of silk may easily be distinguished 

 from those of linen or cotton by the appli- 

 cation of Millon's or Schultze's test, both of 

 which colour the silk, but neither of them 

 the linen or cotton. The test for cellulose 

 is equally applicable to the same purpose. 



SIMULTANEOUS BUNDLES. See 

 VASCULAR TISSUE. 



SIPHOCORY'NE, Buckt A genus of 

 Aphides. (Buckton, Aphid, ii.) 



SIPHONA'CE^E. A family of Confer- 

 void Algse, either marine, freshwater, or 

 growing on damp ground ; characterized by 

 the individual fronds being composed of 

 large branched cells, the contents of which 

 expelled in various forms serve for the 

 reproduction. The fronds mostly have a 

 more or less compound character, either 

 from regular ramification, or by a kind of 

 stoloniferous multiplication at the base of 

 the cells ; and in Hydrodictyon, which seems 

 best placed in this family, the cells are 

 always connected together by their extre- 

 mities, so as to form a net-like frond. In 

 the majority of the genera the cell -contents 

 are green. The modes of reproduction 



exhibit considerable diversity, and are pro- 

 bably still imperfectly known in most of the 

 genera. Codium and Bryopsis are repro- 

 duced by the discharge of the contents of 

 certain cells in the form of numerous small 

 ciliated zoospores. Vaucheria is increased 

 by large elliptical solitary zoospores, covered 

 with vibratile cilia ; in Hydrodictyon the 

 cell-contents are converted into a multitude 

 of ciliated zoospores, which unite to form a 

 new net or frond before leaving the parent 

 cell ; while in Botrydium the cell-contents 

 are said to he discharged in the condition 

 of motionless gonidia ; but we imagine this 

 point is not quite certain. In addition to 

 the gonidial reproduction, spores have been 

 discovered in Vaucheria^ and will probably 

 be found in the rest. In Vaucheria they 

 occur in special branch-cells ; here, however, 

 accompanied by antheridial cells, which pro- 

 duce spermatozoids, fertilizing the sporan- 

 gial cell. Spores have not yet been ob- 

 served in the other genera; but it is to be 

 expected that they will be found in them 

 also. More particular details on the very 

 interesting genera of this somewhat hetero- 

 geneous family will be found under their 

 respective heads. British genera : 



Codium. Filaments green, branched, 

 closely interwoven into a spongiform frond, 

 producing biciliated zoospores in sporangia! 

 cells borne on the sides of the erect clavate 

 branches; marine. 



Bryopsis. Filaments green, free, pin- 

 nately branched, producing two- or fuiir- 

 ciliated zoospores in the extremities of the 

 branches ; marine. 



Vaucheria. Filaments green, more or 

 less branched, continuous, producing in 

 their apices large solitary zoospores covered 

 with cilia ; also bearing lateral globose spo- 

 rangial cells and hook-like autheridial cells 

 or horns; marine or freshwater, and still 

 more commonly on muddy ground, damp 

 garden-pots, &c. 



Botrydium. Frond a spherical green 

 vesicle seated on a ramified filamentous 

 base, the cavity of the whole continuous; 

 the ramified base producing new vesicles, 

 sporanges, by stoloniferous growth. Mul- 

 tiplied by the granular contents of the vesi- 

 cle discharged by a rupture at the summit. 

 On damp, mostly clayey ground subject to 

 floods. 



Hydrodictyon. Frond a green bag-like 

 net, with usually pentagonal open meshes, 

 formed of cylindrical cel-ls connected by 

 their ends. Keproduced by ciliated zoo- 



