RELATIVE POSITIONS OF LATERAL MEMBERS. 
191 
Fig. 149. — Transverse section throng-h the convolu- 
tion of the ieaf-sheaths 1—6 of Sabal icmbraciclt/era ; 
in the centre is a young leaf-blade. The arrangement 
of the leaves is a ? divergence. If the numbers 1 — 6 
are united by a line, the genetic spiral is obtained. 
in simultaneous whorls \ as the petals, stamens, and carpels of most flowers; or 
even in successive whorls where the members are formed in advancing order right 
and left, as in Characeas and the flowers of 
Reseda (Fig. 145). In the successive whorls of 
Salvinia naians the construction of a genetic 
spiral would be equally impossible. Fig. 150, B 
shows the diagram of the stem of this plant 
with three consecutive three-leaved whorls ; in 
each of these the leaf iv is formed first, then 
the leaf Z^, and finally the leaf L,^. If an 
attempt be made to construct the spiral, it 
must pass from w over across to Z^, then 
again in the same direction over w across 
to the figure thus formed is a circle, in 
which the divergences of successive leaves vary 
greatly. If we now pass to the next whorl, the 
line proceeds in a spiral direction to the next 
leafze;; but then, to retain the genetic succes- 
sion in the second whorl, the line must be continued in an opposite direction ; 
and this is repeated with every new whorl. It is evident that no clear conception 
can be obtained in this forced manner, and the whole construction appears alto- 
gether superfluous, since it is 
required by no feature in the 
history of development. The 
stem of this plant is construct- 
ed, as Pringsheim has shown, 
of two rows of segments {G, 
H,J, K, &c., in Fig. 150, Ä), 
which arise alternately right 
and left from the apical cell. 
Even before the production of 
the leaves each segment under- 
goes various divisions, and in 
this manner the stem is built 
up of transverse disks which 
are in alternate succession 
nodes and internodes. Each 
nodal disk consists of the 
anterior half of an older 
segment and the posterior 
half of a segment next younger in age, as shown in the figure. An internode 
is formed of a whole segment of one row and of two half-segments of the other 
Fig. j^o.—A the cone of growth of the stem of Salvinia ftntans, regarded 
diagrammatically and looked at from above; xx projection of the plane which 
divides it vertically into a right and left half; the segments are indicated by 
stronger outlines, their divisions by thinner lines ; the succession of the segments 
is denoted by the letters F—P; B diagram of the stem with three whorls of leaves, 
its ventral side indicated by v v; w the first-formed floating leaf; L\ the aerial leaf 
formed next ; Li the second aerial leaf of the same whorl formed last of all between 
the two first (after Pringsheim). 
^ Many writers employ even in such cases the conceptions borrowed from a spiral arrangement, 
considering arbitrarily as of successive origin the members of the whorl which arise simultaneously; 
but this is not in harmony with a true scientific method. 
