RELATIVE POSITIONS OF LATERAL MEMBERS. 



\6^ 



The lateral members are, on the other hand, isolated or scattered when each 

 member stands on a different zone of the axis. If the surface of an axial structure 

 (which sometimes is quite ideal, as in Aspidiiim Filix-mas, &c.) is imagined to be 

 continued through the base of each lateral member, the section forms its Pla7ie of 

 Insertion. An imaginary point in this is considered its organic centre, but does 

 not usually correspond to its geometrical centre ; this point may be termed the Point 

 of Insertion (cf. sect. 27). A plane which bisects a lateral member symmetrically, 

 or divides it into two similar halves (sect. 28), and contains the axis of growth of 

 the lateral member as well as that of the axial member, passes through the point of 

 insertion, and is called the JSIedian Plane of the lateral member in question. If 

 members are so arranged at different heights on an axis that their median planes 

 coincide, they form a straight row or Orthostichy ; generally there are two, three, or 

 more orthostichies on an axial structure, and the members are then said to be recti- 

 serial. If there are no orthostichies, /. e. if the median planes of all the members 

 intersect one another on an axis without coinciding, their arrangement is solitary. 



The size of the angle which the median planes of two members of the same 

 axis enclose is their Divergence ; it is expressed either in degrees or as a fraction of 



Fig. 138— Diagrams of a slioot witli tlie leaves arranged 

 sinj,'ly with a uniform divergence of ^. 



Fig. 139 —Diagram of the flower-stallc o^ Paris quadrifclia; 

 II whorl of the large foliage-leaves beneath the flower; ap 

 outer, ip inner pcrianth-whorl ; <»tz outer, i.x inner stamens ; in 

 the centre is the rudiment of the fruit consisting of four carpel- 

 lary leaves. 



the circumference of the axis, which is then supposed to be a circle, although in 

 fact this is not usually the case. In order to represent the divergences clearly, they 

 may be drawn on the horizontal projection of the vertical axial structure, in the 

 manner represented in Figs. 138 and 139. The transverse sections of the axial 

 structure which bear the lateral members, in this case leaves, are denoted by 

 concentric circles, and in such a manner that the outermost circle corresponds to 

 the lowest, the innermost to the highest transverse section. On these circles, which 

 thus represent the relative ages from without inwards according to their succession 

 in the acropetal development of the axis, the places of the members are denoted 

 by points, or the forms of the planes of insertion themselves may be approximately 

 indicated, as in the figures. On such a projection or diagram the median planes 



Ellis, Mathematical Tracts.— A. Dickson, Trans. Royal Soc. Edinb. vol. XXVI. p. 505.— Chauncey 

 Wright, Mem. Amer. Acad. vol. IX. p. 37Q.— H. Airy, Proceedings Royal Society, vol. XXI. p. 176. 

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



