Z ARRANGEMENT OF ORGANS. 



modes of arrangement, dependent, as it would seem, on 

 their simultaneous or on their successive development ; 

 thus, if two leaves on opposite sides of the stem are 

 developed at the same time, we have the arrangement 

 called opposite ; if there are more than two, the dispo- 

 sition is then called verticillate or whorled. On the 

 other hand, if the leaves are developed in succession, 

 one after the other, they are found to emerge from the 

 stem in a spiral direction. In either case the leaves 

 are arranged in a certain regular manner, according to 

 what are called the laws of Phyllotaxis, which need not 

 be entered into fully here ; but in order the better to 

 estimate the teratological changes which take place, it 

 may be well to allude to the following circumstances 

 relating to the alternation of parts. The effect of this 

 alternation is such, that no two adjacent leaves stand 

 directly over or in front one of the other, but a little 

 to one side or a little higher up. Now, in the alternate 

 arrangement the successive leaves of each spiral cycle 

 alternate one with another till the coil is completed. 

 For the sake of clearness this may be illustrated 

 thus : Suppose the spiral cycle to comprise five leaves, 

 numbered 1, 2, 3, 4, 5, then 2 would intervene between 

 1 and 3, and so on, while the sixth leaf would be 

 the commencement of a new series, and would be 

 placed exactly over 1 . This arrangement may be thus 

 formularised : 



6 7 8 10 



12 3 4 5 



In the verticillate or simultaneous arrangement of 



leaves the case is somewhat different. Let us suppose 



a whorl of eight leaves, surmounted by a similar whorl 



of eight. In such a case it will generally be found 



