152 



PRINCIPLES OF PLANT-TERATOLOGY. 



terminal male flower, in the case of three inflorescences, 

 which normally consists of two inner smaller and two 

 outer larger petals, and is perfectly horizontal, had 

 became vertical in position and quite irregular, con- 

 sisting of a large, helmet-shaped upper and a smaller 

 spoon-shaped lower petal, which latter was notched at 

 the apex (fig. 122). This case is introduced here as 

 an example of the corolla as a whole changing from 

 the actinomorphic to the zygomorphic condition. 

 There does not, however, appear to be any evidence 

 to show that the individual petals, as in all the in- 

 stances described in the previous section, have under- 

 gone any transformation ; the change seems rather due 

 to the fusion of the two petals respectively in each pair : 

 the two smaller lateral and the two larger median. 



In the so-called " double " Chrysanthemum and 

 Pyrethrurn the change, which has, of course, no con- 

 nection with true doubling (transformation of stamens 

 or carpels into petals), is due to the regular tubular 

 corolla of the disk-florets becoming irregular and ligu- 

 late like that of the ray-florets. We thus see that 

 such capitula assume under abnormal conditions a 

 character which occurs normally in the tribe Ligulse- 

 floras. In some varieties of these two plants the disk- 

 florets, normally small and yellow in colour, may not 

 change to the extent of becoming irregular, but their 

 corolla may simply become much enlarged and white 

 in colour. In those which become ligulate the enlarge- 

 ment of the petals is accompanied by fusion of all five 

 to form the anterior ligula. 



In the groundsel (Senecio vulgaris) the ray-florets 

 have normally a tubular corolla indistinguishable 

 from that of the disk-florets. Occasionally abnormal 

 capitula have been seen in which the ray-florets 

 develop a ligulate corolla ; and this feature is, of 

 course, a normal one in other species, e. g. 8. Jacobdea. 



In certain species of Viburnum, e. g. the guelder-rose 

 (F. OjjiiIus), the outermost flowers of the corymbose 

 inflorescence are, as compared with the inner flowers, 



