6rO 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 '/s or '^/^ 



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, Geraniaceae, Rutaceae, and Zygophyllaceae, 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 Tropaolum, on the 



Fig. 415. — Flower oi Ileracleu7tt piibescens with zygoinorphic corolla. 



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 Flonver. 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. fiir wiss. Bot. vol. VIII, p. 205, 

 ^ Rohrbach (Bot. Zeitg. 1869, Nos. 50, 51) 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, lib. einige Acerineen, Bot, Zeitg. 1861), all the stamens are 

 developed simultaneously in the Acerineoe,] 



