434 Rev. S. Haughton on the General Law of Phyllotaxis 



whorls, and cannot be supposed to be produced in succession, 

 as in alternate-leaved plants. 



Some of the vvhorled-leaved Exogens may be reduced to this 

 law^, such as the simple case of opposite-leaved plants, which is 

 not reducible to the common law of phyllotaxis, as we cannot 

 suppose the two opposite leaves to be produced in succession ; 

 but the great majority of Exogens follow a different law. 



According to all writers on botany, the leaves of alternate- 

 leaved Exogens and Endogens are placed upon the stem at 

 angles represented by the fractions 



1 4 2 a _5_ _a. 15 &■/. 



of an entire circumference. 



In opposite-leaved plants, which is the simplest case of 

 whorled structure, we cannot assign any such law of develop- 

 ment to the leaves, even by calling to our aid the hypothesis of 

 arrested growth, for the leaves succeed each other at intervals 

 of \, ^, alternately, and cannot be reduced to the phyllotaxis of 

 alternate leaves. We should therefore, I believe, assign to all 

 whorled plants a law of phyllotaxis of their own, which is very 

 simple, as already stated. 



The floral envelopes of almost all Exogens and Endogens 

 follow this law of whorled structure, so much so that any devia- 

 tion from it is remarked, and considered due to the suppression 

 of a whorl, as in the case of the Primulacese. It is therefore 

 evident that there must be some mode of passing from one law 

 to the other, as both occur in the same plant. As it is impos- 

 sible to reduce the law of whorled-leaved plants to that of the 

 alternate-leaved plants, I have made some attempts in the oppo- 

 site direction, but have not yet collected sufficient facts to draw 

 any general conclusions. I shall give an example or two. 



Many of the Exogens possess a five-leaved whorled arrange- 

 ment of their floral organs, while the leaves of the stem are 

 arranged alternately at a divergence of ^. This may be deduced 

 from four whorls in the following manner : — Let the alternate 

 whorls be suppressed ; if the remaining whorls were converted 

 into a spiral, they would consist of two spires of five leaves each, 

 with an angle of divergence of J-; but if we suppose the alter- 

 nate leaves suppressed, the two spirals would coalesce and form 

 one, taking two turns round the axis, and containing five leaves, 

 giving an angle of divergence of y. 



If I were at liberty to adopt the Law of Natural Selection, 

 I should say that, no doubt, the })lant found it to its advantage 

 to drop these supernumerary leaves, and so became elevated 

 into the condition of an alternate-leaved plant. 



In the preceding case, in order to deduce the arrangement of 



