Dr. II. Airy on Leaf- Arrangement. 387 



the leaf-order is in perfection, undisturbed by contortions or 

 inequalities of growth ; but as the bud develops 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 iu the bud is 

 disguised in the grown twig. 



In lateral shoots of yew and io.rand 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 uZwi-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 hud we see at once what must be the use of leaf-order. 

 It is for eco)iomi/ of space, whereby the bud is enaliled to retire 

 into itself and present the least surface to outward danger and 

 vicissitudes of temperature. The fact that the order | does not 

 exliibit 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 for- 

 mer. 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 tv.-elve months of its open- 

 air life it is in a state of siege, against which a compact arrange- 

 ment of its embryo-leaves ^vithin 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, 5, f , |, ■^, &c., 

 may be shown by the following experiment : — 



Talve a number of spheres (say oak-galls) to represent embryo 

 leaves, and attach them in two rows in alternate order (g) along 

 opposite sides of a stretched india-rubber band. Give the band a 

 slight twist, to determine 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 g, with three steep spirals. If the spheres are set 

 a little away from the axis, the order becomes condensed into 

 (nearly) f, with great precision and stability. And it appears 

 that further contraction, with increased distance of the spheres 

 from the axis, will necessarily produce the orders (nearly) |, yy, 

 ■^j, &c. in succession, and that these successive orders represent 

 successive maxima of stability iu the process of change from the 

 simple to the complex. 



25* 



