INTEODUCTOEY CHAPTEE. 



3 



directly above the first ; whence it results that, if the leaves completing the 

 spiral (1, 2, 8, 4, 5) -were all placed on a level with the first, they would form a 

 whorl around the stem. This arrangement; is more easily traced on young branches 

 of trees than on herbaceous stems. 



The fibro-vascular bundle connecting the green expansion of the leaf with the 

 stem is the petiole {petiolus, fig. 6). It extends from the axis to the blade or limb 



S. Oak; Branch. 



7. Wallflower, flower. 



{limbus, lamina), which is composed of parenchyma and fibro-vascular bundles, which 

 latter form the nerves {nervi, 1, 2, 3). The middle nerve of the limb, which is 

 continuous with the petiole, is the median nerve or midrib {n. medius, casta media). 

 The bundles which rise from each side of the midrib are the lateral nerves {n. late- 

 rales) ; and these again give rise to secondary (2), tertiary (3), &c. nerves, according 

 to their subdivision. 



A leaf springing directly from the stem without a petiole is sessile (/. sessile, 

 figs. 2-4), and that with a petiole is petiolate (/. petiolatum, figs. 5, 6). The leaf- 

 blade is protected on both surfaces by a thin, colourless, and transparent skin 

 [epidiermis), which covers almost the entire plant, and will be described later. 



The coloured leaves, arranged in whorls at the extremities of the ultimate 

 branches of the axis, together form the flower {flos, fig. 7). The branch which 

 immediately bears a fiower, and forms the axis of its component whorls, is its 

 peduncle or pedicel {pedunculus, pedicellus, fig. 7, Ped). Its more or less swollen 

 extremity, upon which the whorls of the flower are grouped, is the receptacle 

 [receptaculum, fig. 10, e). 



In the most fully developed plants the flower is usually composed of four 

 successive whorls (fig. 7), of which the internodes are suppressed. The outer 

 or lower whcrl is the calyx {calyx, figs. 7, s, and 8), the leaves of which are 

 sepals {sepala, fig. 8). The whorl within or above the calyx is the corolla {corolla, 

 fig. 7, P), and its leaves- are petals {petala, fig. 9). When a petal is not sessile, but 

 bas its blade (l) borne on a petiole (o), this petiole is called the claw {amjuis). 



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