274 



BOTANY OF THE LIVING PLANT 



succession of parts, excepting the reduced calyx, is acropetal, and the carpels 

 are actually nearest to the apex. Thus the epigynous state results from the 

 relatively slow growth of the apex, which is overtopped by the stronger growth 

 around it. 



(vii) Differences of Symmetry. In many flowers, such as the Butter- 

 cup, Rose, or Tulip, the parts may develop equally on all sides of the 

 axis, giving a radial or actinomorphic symmetry. This is believed to 

 be a primitive condition, and it is found in flowers which are not 

 highly specialised. It prevails in spiral or hemicyclic types. In 

 others the development may be unequal on different radii, so as to 

 give the flower a lop-sided form ; but almost always so that it can 

 be divided by at least one plane of symmetry into two equivalent 



ABC 



Fig. 199. 



Diagrams of zygomorphic flowers. A, of Aconite (Ranunculaceae). B, of the 

 Laburnum (Leguminosae). C, of Toadflax (Scrophulariaceae). (After Eichler.) 



halves. Such flowers are called dorsiventral, or zygomorphic, and the 

 Aconite, Laburnum, and Toadflax are examples (Fig. 199). The plane 

 of symmetry is usually the median plane ; but sometimes it is oblique, 

 as in the Horse Chestnut ; or transverse, as in Corydalis. Zygomorphy 

 is very common, it occurs in most families, and it is characteristic of 

 some throughout, as in the Orchidaceae, or Labiatae. Its importance 

 lies in relation to the transfer of pollen by animal agency. The most 

 perfect mechanisms for this end are found in zygomorphic flowers. 

 It is a derivative or specialised state, and comparison shows that it 

 has been acquired by modification from the radial state, along many 

 distinct lines of descent. 



The Biological Specialisation of the Flower. 



Diversity of floral structure is very fully illustrated in the Angio- 

 sperms. But however varied, or sometimes even extravagant the 

 structure of flowers may appear to be, each flower should, in point 



