Gibb : The Study of a Fircone. 387 



-the stem. Here three coils are made, and eight leaves passed 

 before one (the ninth) is found growing exactly above the first. 

 ■Carrying the eye up, it will be observed that exactly the same 

 order prevails in every series of eight leaves, the seventeenth 

 and twenty-fifth standing in a direct line above the first, and 

 beginning respectively a new series. 



Here instead of the | arrangement we have then the 



and the regular rhythm of coil and leaf is not 3, 2 ; 3, 2 ; as 

 with the Piitosporium and the rose, but 3, 3, 2 ; 3, 3, 2 ; a 

 simple enough little bird-note still. 



These rhythms of the plant world are very fascinating to 

 search out, but they are often by no means easy to discover, 

 as the}' are masked by many circumstances. Sometimes there 

 is suppression of certain leaf buds which throws out all the 

 series ; again there may be suppression of internodes between 

 the leaves, and two leaves which would normally be separated 

 b}^ a space of stem may grow from the same joint in a pair. 

 The common laurel when growing in its usual spreading way 

 with its branches horizontally extended, has adopted the habit 

 of arranging its leaves alternately so that the third leaf stands 

 above the first, and the series appears to be completed in one 

 coil passing through two leaves. This is very confusing ; but 

 if the investigation is pursued it is soon discovered that young 

 sturdyshoots growing perpendicularly, have their leaves arranged 

 all round the stem (instead of all facing one way on each side 

 of it), and that now the sixth leaf is placed above the first, 

 and the coil runs through three and two leaves alternately, 

 shewing the -| arrangement. The reason for the other plan is 

 obviously to obtain full sunshine and air for every leaf on the 

 trailing horizontal branches. If they had kept to their normal 

 manner of growth, certain leaves would have been pointing 

 towards the earth. During the transition from one arrange- 

 ment to the other, the placing of the leaves is often extremely 

 confusing. This case serves as a good example of the ways in 

 which leaf-arrangement may be modified by circumstances. 

 It is interesting to remark here that upon the numbers which 

 make up the series of leaves as arranged on the stem, depend 

 the numbers of petals and sepals (or transformed leaves) in 

 the flower. Thus the holly, with its sequence of eight leaves, 

 has four sepals and four petals, four stamens and four divisions 

 in its pistil ; that is its flower is composed of two leaf-series 

 compressed into four whorls of changed leaves — sepals, petals, 



1909 Nov. I. 



