AN ENCYCLOPEDIA OF HORTICULTURE. 



407 



Sedum continued. 

 cymed. L alternate, ascending, tapering at both ends. Stem 



(Rhone). JL pink; . 

 L alternate, spreading, tapering to the 



cymes corymbose, 



globose, long-stalked, 

 fuse and apex, coarsely toothed. 

 S. T. rubella (reddish). JL pink ; cymes globose, the lower 

 ones on long, horizontal stalks, forming an elongated, oblong 



rarida 



toothed. 



L red, ta 



tapering to the base, narrow-oblong, coarsely 



d). JL greenish or yellowish, Jin. 



in diameter ; cymes numerous, globose, many-flowered, on long, 

 SHertiivalin^ forming a large, compact panicle. August and 



S . T . t h 



ocifkcmuci. L opposite or whorled; lower ones ascending, 3in. 

 by lin., oblong, acute, toothed in the upper two-thirds ; upper 

 ones smaller, sub-cordate. Stems about 2ft. high, reddish. 



other forms of S. Tdephium hare been mentioned : 



white, Jin. across, four-parted: 

 oblong, acute, twice the length of 

 the sepals ; flower-stems erect. July and August L Jin. long, in 

 whorls of three, sub-orbicnlar-spathulate, crowded into rosettes 

 at the ends of the barren stems ; those of the fertile stems 



scattered, ascending or spreading, oblong, acute. 

 6in. to Sin. long, prostrate. North America, 1789. A 

 or glaucous evergreen. (B. M. 1977 ; B. R. 142.) 

 >. trifldnm (trifid). jL at first reddish, in small, dense, terminal 

 cymes ; petals linear-lanceolate, twice the length of the lanceolate 

 sepals. Summer. ' ' ' 



or pinnatifid. 

 Temperate Himalayas. 

 S. nmbillcoides (Umbilicus -like). JL white, seven-parted; 

 inflorescence long-stalked, cymose. I. rosulate, convex, oblong- 

 lanceolate. Alatan, Turkestan. Evergreen. (R. G. 917.) 



SEED. Seeds are met with only in flowering plants, 

 and are the result of the union of the male and female 

 elements, for the reproduction of the species. They 

 are the ovules, fertilised by the action of the pollen. 

 and subsequently ripened (see Ovule) ; and each con- 

 tains an embryo (rarely two or more), i.e., a young 

 plant, capable, when the Seed is placed in suitable con- 

 ditions, of becoming, in all points, like its parents. The 

 existence of an embryo in Seeds affords a sharp distinc- 

 tion between them and the spores of Cryptogams, which 

 correspond with them in function, but are very often 

 one-celled, and never contain an embryo. A reference 

 to the account given under Ovule will facilitate the 

 explanation of the changes that the ovules undergo in 



FIG. 466. ORTHOTROPOUS OVULE n SECTION, showing two Coats 

 (o, 6), with the Micropyle (e\ the Nucellus (e), and the Em- 

 bryo-sac (d). The signification of the other letters is as follows : 

 / is the Funicnlos or Stalk ; g, the Baphe, or Fibro-vascular 

 Bundle in the Stalk ; A, the Cbalaza, where the Stalk after- 

 wards breaks off, and leaves the Hilum or Scar ; i, the Placenta 

 from which the Ovule grows. 



becoming Seeds. In the unfertilised ovule (see Figs. 466 

 and 467), there are generally two coats, surrounding a 

 central cellular mass called the nucellus ; but there is 

 a passage (micropyle) at the true apex of the ovule, 

 through both coats. In the middle of the nucellus lies 

 a large cell, the embryo-sac, formed by the union of two 

 or more cells. Within this sac are several cells (see Fig. 

 468), vis., at the end next the micropyle two oval or 

 elongated cells, the " helper cells," with a round cell (two in 

 Santalum album) at their inner end, called the embryonal 



Seed-continued. 



vesicle. At the opposite end of the embryo-sac are three 

 others, called antipodal cells, which are believed to be 

 the hut traces of a prothallus, such as is formed in 

 Selaginella among Vascular Cryptogams. Among the 

 protoplasm that lines the walls of the embryo-sac as a 

 thin layer, two nuclei may generally be distinguished. 

 After fertilisation, i.e., after the pollen tube has reached 

 the embryonal vesicle through the micropyle, the helper 



FIG. 467. ANATROPOUS OVULE OF ACT.EA SPICATA, i.x SKCTIOJI 

 Only one Coat (a) exists on it (other lettering as in Fig. 466). 



cells and the antipodal cells disappear; the cell wall 

 of the embryonal vesicle becomes thicker, and the cell 

 grows, and becomes divided by cross-walls. At last it 

 forms a cellular mass, the embryo, on the end of the 

 suspensor, which part in most plants is a row, but in 

 some is a mass, of cells. In the embryo of the ripe Seed, 

 it is generally easy to make out radicle, stem, plumule, 

 and one or two cotyledons. Tlins : in it all the vegetative 



K 



FIG. 468. EMBRYO-SAC OP ANGIOSPER* BEFORE FERTILISATION, 

 IN THREE STAGES, showing (A) Helper Cells, ( ) Embryonal 

 Vesicle, (a) Antipodal Cefls, and (n, n) Nuclei. In A, the 

 Helper and Antipodal Cells, and the Embryonal Vesicle, an 

 still angular, and the two Nuclei are wide apart ; in B. the 

 Embryonal Vesicle is rounded, and the Nuclei lie don to- 

 gether ; in C, the Embryonal Vesicle is ready to be fertilised, 

 the two Nuclei are united to form one, and the Antipodal, as 

 well as the Helper Cells are rounded off. 



organs of the future plant are represented, except in a 

 few families (e.g., Orchids), in which the embryo is 

 only a mass of cells, without distinction of parts. In 

 Coniferce, the course of development of the ovule into the 

 Seed is different in several respects; but the result is 

 much the same as regards the structure of the mature 

 Seed. While the embryo is in course of development, the 

 nuclei of the embryo-sac give rise to the endosperm. At 

 first, this consists of cells free from one another, but soon 



