1 



56 



LEAVES. 



their relative lateral position; and this is true ofall modifica" 

 tion of leaves. Did we know, what concurrent circumstan- 

 ces were required for the development of a leaf, we might 

 then perhaps find a reason for these variations. But there is 

 not only a disposition of the leaves, but ofall vegetable pro- 

 ductions to assume a spiral arrangement. The stems of 

 plants take, in ahuost all cases, more or less of the spiral 

 growth however straight the stem may be. We may see this 

 also in the epidermis of various trees and more particularly 

 in the stems of twining plants as the hop and ivy. 



The Sj)iral arrangement of leaves has excited mucti interest 

 recently; and Braun, a German Naturalist has applied Math- 

 ematical formula to express the elements of the spires in dif- 

 ferent species, and for determining their constitution. The 

 most simple parts only of his memoir are admissable to an ele- 

 mentary work, and we shall state a few of his principles as 

 quoted by Lindley. "All the spires depend upon the position 

 of a fundimental series, from which the others are deviations. 

 The nature of the fundamental series is expressed by a frac- 

 tion, of which the numerator indicates the whole number of 

 turns required to complete one spire, and the denominator the 

 number of scales or parts that constitute it. Thus 8-21 in- 

 dicates that eight turns are made round the axis before any 

 scale or part is exactly vertical to that, which was first 

 formed, and the number of scales or parts that intervene be- 

 fore this coincidence takes place is 21," which occurs in the 

 Corylus, Piantago lanceolata. 2-5 expresses that the leaves, 

 buds or scales make two turns, bc^fore a leaf, scale, or bad is 

 exactly above t!ie one from which we start and that there are 

 five of them. This is the most common variety. Cherry, 

 Althea, Potatoe, Peacli, 6lc. are of this variety. J includes 

 the spikes of the grains. 3-8 includes the Bay, Holly, (fee. 

 Prof. Lindly remarks that, "it does not, however, appear that 

 this enquiry lias led to any thing beyond the establishment of 

 the fact that beginning from the cotyhdons the whole of the 

 appendages of the axis of plants — leaves, calyx, corolla, 

 • stamens and carpels — form an unirsterrupted spire, governed 

 by laws whic'.i are nearly constant." 



Conjectures have been made, as to the cause of the uniform 

 ' arrangement of leaves on the stem, and why they take one 

 form in any given species rather than another, but they are 

 so completely C(jnjectural that we shall not occupy space in 

 stating the'U. Oar own observations have not afforded us 

 the lea^t clue to the solution of the problem. 



62. Leaves usually present surfaces of different appearance/ 



