DEVELOPMENT OF THE MEMBERS OF A BRANCH-SYSTEM. 



187 



Fig. 144.— Apical region of a shoot of Coriaria mrrtifolta ; A in 

 transverse section, B in longitudinal section; s apex of the stem; bb 



in decussate whorls of twos ; k axillary bud ; 



which lateral shoot when we have sympodial pseud-axes. Sometimes, however, distor- 

 tions occur {e.g. in Solanacese) which might lead to erroneous conclusions if reference 

 were not made to the earliest stages of development. 



Sect. 26. The Relative Positions of Lateral Members on a Common 

 Axis\ — In order to bring the facts which we have now to consider into a clear 

 and simple arrangement, it is necessary, in the first place, to explain the use of a 

 few technical expressions and geometrical modeg of representation. 



By the term Axial Structure or Axis is to be understood, in future, when the 

 contrary is not expressly stated, any member that continues to grow at its apex 

 and produces lateral members ; for example, a mother-root with its lateral roots, 

 a stem with its leaves ^ the mid-rib 

 of a leaf with its leaflets, pinnae, 

 or lobes, or a thallus-shoot with its 

 lateral outgrowths. 

 ^ If two or more similar lateral 

 members proceed in different direc- 

 tions from the same transverse zone 

 of an axis, they constitute a Whorl. 

 A true whorl results when the zone 

 of the axis which produces it is 

 always at right angles to the axis 

 (Fig. 136); a Spurious or Pseudo- 

 Whorl when the zone is the result 

 of unequal development of the axis, 

 or when lateral members which 

 were formed at the same level 

 have become so far separated by 

 subsequent unequal elongation of 

 the axis, that they appear, in the 

 mature state, distributed into different 

 zones. Simultaneous Whorls are those 

 whose members are formed simul- 

 taneously (Fig. 144). Whorls are 

 successive when the members at the 

 same zone grow in succession either 

 right and left, as is shown in Fig 145, and as occurs in the true leaf-whorls of 



^ Roper, Linneea, 1827, p. 84. — Schimper-Braun, Flora, 1835, pp. 145, 737, 748. — Bravais, 

 1. des Sci. Nat., vol. VII. 1837, pp. 4^» I93- — Wichura, Flora, 1844, p. 161. — Sendtner, Flora, 

 1847, pp. 201, 217. — Brongniart, Flora, 1849, p. 25. — Braun, Jahrb. fur wissen. Bot. vol. I. 1858, p. 

 307._Irmisch, Flora, 1851, pp. 81, 497. — Hanstein, Flora, 1857, p. 407.— Schimper,ditto, p. 680. — 

 Buchenau, Flora, i860, p. 448. — Stenzel, Flora, i860, p. 45. — Numerous papers by Wydler, e. g. 

 Linnoea, 1843, p. 153; Flora, 1844, 1850, 1851, 1857, 1859, i860, 1863, and elsewhere. — Hofmeister, 

 Allgemeine Morphologie der Gewebe, §§ 8, 9. [Haughton, Manual of Geology. — Ellis, Mathe- 

 matical Tracts.— A. Dickson, Trans. Royal Soc. Edinb. vol. XXVI. p. 505.— Chauncey Wright, 

 Mtm. Araer. Acad. vol. IX. p. 379. — H. Airy, Proceedings Royal Society, vol. XXI. p. 176. — 

 Beal, American Naturalist, 1873, vol. VII. p. 449.] 



'^ [The term Phyllotaxis is used in works on descriptive botany to denote the mode of arrange- 

 ment of leaves, and especially of the foliage-leaves on the stem.] 



leaves in pairs, t. 

 g youngest vessel. 



Fig 145.— Development of the flower of the mignonette (after Payer) ; 

 A a younger, B an older bud ; from the latter the anterior sepals s have 

 been removed, the posterior ones left; // petals; j/ stamens, the pos- 

 terior ones already large, the anterior ones not yet even in a rudimentary 

 state ; c the rudiment of the pistil. 



