240 



BOTANY OF THE LIVING PLANT 



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 

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

 Aconite, Laburnum, and Toadflax are examples (Fig. 189). 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 



FIG. 189. 



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

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



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. 



Under these seven headings the most important factors are included 

 which lead to that diversity of floral structure which is so fully illus- 

 trated in the Angiosperms. Examples are described in detail in 

 Appendix A, to which reference should be made. Each individual 

 flower may, in point of final analysis, be regarded as a simple shoot 

 bearing sporangia. Structurally its shoot-nature is evident in the 

 less specialised types, though in the more specialised it may be so 

 hidden by peculiarities of form as to be recognisable only after careful 

 analysis and comparison. The several factors are effective in the most 

 varied combinations, so that the analysis which is required to reduce 

 a complicated structure to terms of construction like that of the leafy 

 shoot may demand both skill and insight. The most obvious end 

 served by the mechanisms that result is the transfer of pollen from the 

 stamen to the carpel. This is a necessary step in normal reproduction. 



