1874.] Dr. H. Airy on Leaf- Arrangement. 21J9 



more rarely in the following (B) : 

 1123 5 8 13 o 



s' i' f iv rs' 29* i7' &c ' ; ( B > 



very rarely in the following (C) : 



1 . 1 2 3 5 & , r 



4' 5' 9' 14' 23' * \f?l 



besides a few isolated values, ~, , L &Q which would find 



11 lo o 



place in higher series. (Hof meister, * Allgemeine Morphologie der 

 Grewachse,' p. 449. Leipzig, 1868.) 



Dealing first with the phenomena of alternate leaf-order, the theory is 

 advanced that, in each of the series A, B, C, &c., the higher orders have 

 been derived from some lower order of the same series by a process of 

 condensation advantageous to the species in which those higher orders are 

 found ; that the scene of this condensation of leaf -order has been the 

 bud and other close-packed forms of plant-growth ; and that the imme- 

 diate gain has been better economy of space. 



In support of this theory it is argued, first, that the use of leaf -order 

 is to be found in that stage of the life of a shoot in which the leaf -order 

 is most regular and perfect. Leaf-order is seen in perfection in close- 

 packed forms of plant-growth, such as the bud, the 6w?6,the radical rosette, 

 the involucre, the composite head, the catkin, the cone, even the seed itself. 

 Therefore it must be in these forms that leaf-order is especially useful. 

 In elongated shoots, on the contrary, with long internodes and distant 

 leaves, the leaf-order has a tendency to lose that regularity which it 

 enjoyed in the bud, and is often disarranged by a twist of the stem 

 or by contortion of the leaf-stalks (required for the better display of the 

 leaf -blades to the light). The native arrangement of the leaves (excluding 



the order ^\ is often a positive disadvantage to them in lateral twigs. 



It is only in the more vertical and unembarrassed shoots that the leaf- 

 blades remain content with their distributive position. Indeed, one chief 

 use of the leaf-stalk seems to be to enable the leaf -blade to make the best 

 of an unfavourable birth-place. (Yew, silver fir, box, and privet are in- 

 stanced as examples.) Hence it appears probable that the use of leaf- 

 order is not to be found in the elongated shoot. 



Looking, then, to the above-mentioned close-packed forms of plant- 

 growth as the scene of the usefulness of leaf -order, it is seen that the cha- 

 racteristic feature which distinguishes them from the elongated forms is 

 contact between neighbouring leaves (or shoots). The whole surface of the 

 stem is occupied by their bases, and no vacant interstices are left between 

 them. It is plain that the process of cell-growth has resulted in great 

 mutual pressure between neighbouring leaves and shoots. Recognizing 



