1873.] Dr. H. Airy on Leaf- Arrangement. 



177 



But first, what are the uses of these orders ? and at what period of the 

 leaf's life does the advantage of leaf-order operate ? The period must be 

 that at which the leaf -order is most perfect : not, therefore, when the 

 twig is mature, with long internodes between the leaves, but while the 

 twig and its leaves are yet in the bud ; for it is in the bud (and similar 

 crowded forms) that the leaf-order is in perfection, undisturbed by con- 

 tortions or inequalities of growth ; but, as the bud developes into the 

 twig, the leaves become separated, the stem often gets a twist, the leaf- 

 stalks are curved and wrung to present the blades favourably to the light, 

 and thus the leaf-order that was perfect in the bud is disguised in the 

 grown twig. 



In lateral sh oots of yew and box and silver fir we see how leaves will 

 get their stalks twisted to obtain more favourable exposure to light ; and 

 if general distribution round the stem were useful to the adult leaves, we 

 should expect the leaves of a vertical <?Zm-shoot (for example) to secure 

 such distribution by various twists of stalk and stem ; but the leaf -blades 

 of the elm keep their two ranks with very great regularity. This goes 

 to show that it is not in the mature twig that the leaf-order is specially 

 advantageous. 



In the bud we see at once what must be the use of leaf-order. It is 

 for economy of space, whereby the bud is enabled to retire into itself and 

 present the least surface to outward danger and vicissitudes of tempera- 

 ture. The fact that the order \ does not exhibit this advantage in any 

 marked degree, supports the idea that this order is the original from 

 which all the more complex spiral orders have been derived. 



The long duration of the bud-life as compared with the open-air life 

 of the leaf gives importance to the conditions of the former. The open- 

 air life of the bud is twelve months, and adding the embryo life of the 

 bud, we have about a year and a half for the whole life of the bud ; and 

 for the twelve months of its open-air life it is in a state of siege, against 

 which a compact arrangement of its embryo-leaves within must be of 

 great value. But the open-air life of the unfolded leaves is (except in 

 evergreens) not more than six months. 



That the order \ would under different degrees of contraction (with 

 twist) assume successively the various spiral orders that exist in nature, 

 in the order of their complexity, ^, j-, §, ^ &c, may be shown by the 

 following experiment : — 



Take a number of spheres (say oak-galls) to represent embryo leaves, 

 and attach them in two rows in alternate order (|) along opposite sides 

 of a stretched india-rubber band. Give the band a slight twist, to deter- 

 mine the direction of twist in the subsequent contraction, and then relax 

 tension. The two rows of spheres will roll up with a strong twist into 

 a tight complex order, which, if the spheres are attached in close contact 

 with the axis, will be nearly the order 3, with three steep spirals. If the 



