SEGESTRELLA. 



[ 573 ] 



SENECIO. 



Parnassia. Monotropa. 



Drosera. Hydrangea. 



Orchis. Saxifraga. 



Pyrola. 



The testa or outer skin of some of the 

 latter (also Begonia), when removed from 

 the seed and viewed with a high power, ex- 

 hibits elegant pitted cells. The surface of 

 the seed of Cobcea is mealy with little scales 

 consisting of pyriform cells containing a 

 spiral fibre (PL 21. fig. 20). 



The surface of various seeds, such as Col- 

 lomia, Ruellia (and the pericarp of many 

 seed-like fruits, such as that of Salvia, Se- 

 necio), present remarkable forms of HAIRS. 



The * stones' of plums or cherries, the 

 so-called shell of the Cocoa-nut and similar 

 fruits, exhibit remarkably thick SECONDARY 

 DEPOSITS. 



The examination of the structure of ripe 

 seeds is a matter of great importance in 

 botany. The investigation will vary much 

 according to circumstances. Where seeds 

 are large, the microscope is only required 

 for the examination of their tissues, but small 

 seeds must be examined by dissection with 

 needles under the simple microscope, or by 

 sections, which are most easily made by fix- 

 ing the softened seed into a piece of wax. 

 Seeds have two coats, the testa and tegmen, 

 or external and internal membrane, and, ac- 

 cording as the seed is or is not albuminous, 

 an albumen enclosing the embryo, or an 

 embryo of larger size immediately invested 

 by the coats. The characters of the ALBU- 

 MEN and EMBRYO will be found under these 

 heads, as also other particulars underOvuLE. 

 Embryos are either Monocotyledonous or 

 Dicotyledonous; sometimes, however, the 

 two cotyledons are soldered together more or 

 less completely ; in the Coniferae and certain 

 genera of Dicotyledonous Angiosperms, as 

 Schizopetalum, the cotyledons appear to be 

 four, six, or more in number ; but the recent 

 observations of M. Duchartre go to show 

 that there exist only two, bifid, trifid, or 

 multifid cotyledons. In other cases, as in 

 Orchis, the embryo remains imperfectly de- 

 veloped, and appears as a mere cellular mass 

 in the ripe seed before germination ; this is 

 destitute of albumen, but in Orobancke an 

 amorphous embryo is found imbedded in the 

 albumen. 



BIBL. General works on Botany. 



SEGESTRELLA, Fr. A genus of Verru- 

 cariese (Angiocarpous Lichens), containing 

 one doubtful British plant, the Lecanora 

 thelostoma of the Brit. Fl. 



BIBL. Leighton, Brit. Angioc. Lichens, 

 p. 34; Hook. Brit. Fl. ii. pt. 1. p. 189. 



SEIROSPORA, Harv. A genus of Ce- 

 ramiaceae (Florideous Algae), containing one 

 rare species, S. Griffithsiana, a little crimson 

 feathery sea-weed, composed of single arti- 

 culated tubes, the joints of which are tra- 

 versed by articulated filaments. The spores 

 are unknown, but the tetraspores, which 

 serve to distinguish this plant from the Catti- 

 thamnia, occur in terminal beaded strings, 

 being formed out of the ramuli. 



BIBL. Harvey, Brit. Mar. Alg. p. 170. 

 pi. 23 C. 



SELAGINELLA, P. de Beauv. A genus 

 of Lycopodiaceae, distinguished from Lyco- 

 podium by the presence of two kinds of 

 spores and the dissimilar habit. This genus 

 includes only one of our native Club-mosses, 

 S. spinosa (Lye. selaginoides) ; but most of 

 the so-called Lycopodia, now so extensively 

 cultivated inWardian cases, fern-houses, &c., 

 belong to this division (fig. 434. p. 405). 

 The principal particulars relating to these 

 plants, especially the remarkable history of 

 the reproduction by the spores, are given un- 

 der LYCOPODIACE^:. 



BIBL. See LYCOPODIACE^J. 



SELENITE. This well-known mineral 

 substance consists of crystallized hydrated 

 sulphate of lime. Its crystals belong to the 

 oblique prismatic system; and the colours 

 exhibited by thin laminae, into which they 

 may be easily split, are very beautiful under 

 polarized light. Polarizing crystals and or- 

 ganic substances, in which the thickness is 

 not suited to the production of distinct co- 

 lours under the polariscope, may be made to 

 exhibit them by placing a plate of selenite 

 beneath the object. For this purpose the 

 plate is usually kept mounted in Canada 

 balsam. 



BIBL. That of POLARIZATION. 



SELIGERIA. A genus of Leptotricha- 

 ceous Mosses, including certain Weissice and 

 Gymnostoma of authors. 



SELLIGU^EA, Bory. A genus of Gram- 

 matideae (Polypodaeous Ferns). Exotic. 



SENDTNERA, Woods. A genus of Jun- 

 germannieae (Hepaticaceae), mostly tropical, 

 one species of which, S. (Jung.} Woodsii, 

 occurs rarely in the mountains of the S.W. 

 of Ireland (devoid of fruit). 



BIBL. Hook. Brit. Flor. ii. pt. 1. p. 126, 

 Brit. Jung. pi. 66 ; Ekart, Synops. Jung. pi. 

 12. fig. 108 ; Endlicher, Gen. Plant. Supp. 1. 

 No. 472-16. 



SENECIO. The surface of the achenia 



