6io 
PHANEROGAMS. 
arise in acropetal order at different heights, i.e. approaching the centre of the flower 
with each step in the divergence, the arrangement is a spiral one. The last appears to 
be actually the case in many calyces ; but it is doubtful whether it ever occurs where the 
angle of divergence of the sepals is Vs or '^s* 
We must now refer again to the cases already mentioned, where new members of a 
whorl are formed between those already in existence and at the same height^. In the 
Oxalideae, Geraniacese, Rutaceae, and Zygophyllacese, an entire whorl of five stamens is 
thus interposed between those already present; according to Payer, in Peganum Harmala, 
a whorl of ten stamens is even formed in this manner, arising, not in, pairs between the 
first five, but lower down at the bases of the petals ; whether the later formed stamens 
arise on the same level with the first, or lower down, is obviously regulated according to 
the space afforded by the changes of form of the growing receptacle. A still further 
departure from the ordinary process occurs in the Acerineae, Hippocastaneae, and Sapin- 
daceae, where Payer asserts that a whorl of five stamens is first of all formed alternating 
with the corolla, in which an imperfect whorl of two or four stamens is subsequently 
interposed at the same height, as is shown by his illustrations. In TropcBolum, on the 
other hand, according to Payer and Rohrbach ^, three stamens first of all appear after 
the formation of the petals, and then between them five others, the distance of which 
from the centre of the flower is however rather greater than that of the three earlier 
ones. 
5. Symmetry of the Flocwer. If the observations which will be found on pp. 187 et seq. 
under the head of General Morphology are now applied to the floral shoot, it is seen 
that true symmetry and distinctly bilateral structure occur here far more commonly 
^ Compare also on this point Pfeffer, Jahrb. für wiss. Bot. vol. VIII. p. 205^ 
2 Rohrbach (Bot. Zeitg. 1869, Nos. 50, 5r) however gives a different explanation to these obser- 
vations from that mentioned here. The equal or greater distance at which the later stamens arise 
from the centre of the flower is a distinct proof that one cannot in this case suppose that the parts 
are produced in a spiral arrangement advancing from without inwards. [See note on p. 601. 
Assuming the correctness of Payer's observations, these are instances of incomplete obdiplostemony. 
According to Buchenau (Morph. Bemerk, üb. einige Acerineen, Bot. Zeitg. 1861), all the stamens are 
developed simultaneously in the Acerine?e.] 
Fig. 413. — Flower oi Heracleum piihesrens with zygomorphic coroll; 
