i88 
MORPHOLOGY OF MEMBERS. 
Chara ; or in a different order, as in the true leaf-whorls of Salvinia {vide injra), 
and in the three- or five-parted calyces of most Phanerogams. 
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 imaginary, as in Nephr odium Filix-mas, &c.) is supposed 
to be continued through the base of each lateral member, the section forms its Piane 
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 (see Sect. 27). A plane which bisects a lateral member symmetrically, 
or divides it into two similar halves, 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 Median 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, i. e. if the median planes of all the members intersect one another 
on an axis without coinciding, their arrangement is solitary. 
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 
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 a horizontal section of the vertical axial structure, in the manner 
represented in Figs. 146 and 147. The transverse sections of the axial structure 
which bear the lateral members — in this case leaves — are denoted by concentric 
circles, the outermost circle corresponding 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 develop- 
ment of the axis, the positions of the members are denoted by dots, 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 of the members 
