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JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



point, that our knowledge has in recent years advanced, and it is with 

 this side of the subject that I shall mainly deal in the following 

 pages. 



To give a precise definition of what is meant by a double flower 

 is by no means easy, for the term is commonly applied to flowers 

 showing very various structural abnormalities. Almost all double 

 flowers, however, have this one feature in common, that the number 

 of petal-like structures present is in excess of the normal. This 

 increase in the number of " petals " may be brought about in several 

 ways. The late Dr. Mastees, in his work on Vegetable Teratology, 

 groups the various causes leading to the condition popularly termed 

 double under the following heads : 



(1) Petalody = petaloid modification of structures ordinarily present 



in the flower, but not normally resembling petals. In this 

 case the number of members is not increased. (See fig. 169.) 



(2) Augmentation in the number of petals as a result of 



augmentation in the total number of structures present. 

 (See fig. 170, d.) 



(3) Isolation of organs ordinarily united. 



(4) Prolification (proliferation) = the formation of buds within the 



flower. (See fig. 175.) 



(5) Dissection o'f the petals, and the formation of outgrowths. 



Of these the two first-named are by far the most frequently met 

 with, and we may confine our attention for the moment to examples 

 of these two classes. In the first case — that of simple petalody, we 

 are concerned with the development of petal-like structures in place of 

 (in most cases) either stamens or pistil or both. In cases belonging 

 to the second class the increase in number of the petals may be 

 real, due to actual multiplication of the number of whorls, or only 

 apparent, the augmentation resulting from the splitting in various 

 ways O'f the original primitive rudiments which are present in their 

 normal number. 



Though in the abstract we may thus conveniently distinguish the 

 morphological causes producing doubleness we find that in practice it 

 is not always easy to ascertain with which we are dealing, for the true 

 nature of the cause can often be determined only by investigation at 

 an extremely early stage in development. The difficulty becomes 

 greater still in cases, not uncommon, where a species produces doubles 

 of more than one type, and again in the numerous instances where two 

 distinct processes are in operation in the flower at the same time, as, 

 e.g., where petalody is combined with a greater or lesser amount of 

 splitting. 



A combination of petalody with some amount of splitting seems 

 in fact to occur much more frequently than either simple petalody or 

 than mere increase in the number oi parts. 



As an instance of simple petalody we may take the case of the 

 Columbine (Aquilegia) (fig. 169). The fact that the flower of Aquilegia, 



