l] PHYLLOTAXY 5 



of scattered leaves is so common and striking that much 

 harmless ingenuity has been expended in reducing the 

 different cases to statistics, while enthusiasts have gone 

 further and tried to enunciate generalisations to which the 

 name of laws has been given ; somewhat too rashly, since 

 the principles underlying the position of the leaves are 

 much more fundamental than these so-called "laws of 

 phyllotaxy" admit, and have to do with the space-rela- 

 tions and pressures in the bud, and the need for exposing 

 the leaves in the best manner to light and air. 



As mere facts of organ ography, however, a few of the 

 commoner cases are useful. 



Thus, if we join the insertions of the successive leaves 

 of the lateral branches of the Lime, Elm, Beech, and 

 many other plants, we find that in passing as above de- 

 scribed from a given leaf to the one vertically above (or 

 below) it, our line describes a spiral which goes once round 

 the axis, and touches two leaf-insertions on the way, in 

 addition to the one started from. In other words, the leaf 

 vertically above or below the chosen one is the next but 

 one, and the fact can be shortly expressed by the conven- 

 tional formula ^. 



This kind of phyllotaxy is termed distichous, because 

 the leaves can evidently be regarded as inserted in two 

 vertical rows, one on each of two alternate sides of the 

 stem. 



If we had chosen one of the Cyperacese (Sedges) or 

 the Alder, Birch, and some other plants, the spiral line 

 drawn from any leaf to the one vertically above it would 

 again pass once round the axis, but go through three leaf- 

 insertions on the way, a fact expressed by the formula J, 

 and the phyllotaxy is termed tristichous, the term ortho- 

 sticky being employed to denote the vertical series, or 

 rank, of leaves. With most branches of the Oaks, Willows, 



