1873.] 



Dr. H. Airy on Leaf- Arrangement. 



179 



variation in that group and in elm, beech, &c, support the view that this 

 order is the original of the spiral orders. 



In many plants we find actual transition from the order \ to an order 

 more complex, as, for instance, in Spanish chestnut, laurels, nut, ivy; 

 and these instances agree in presenting the complex order in the buds 

 that occupy the most exposed situations, while they retain the simple \ 

 in the less exposed lateral buds. Several kinds of aloe have the order j 

 in their basal leaves and a higher order in the remainder. A species of 

 cactus often contains a complete epitome of phyllotaxy in a single plant 

 or even in a single shoot. 



Shoots of acacia often present a zigzag disposition of their leaves, on 

 either side of the branch, which seems unintelligible except as a distortion 

 of an original two-ranked order. 



The prevalent two-ranked arrangement of rootlets or roots seems to be 

 a survival underground of an order which originally prevailed through 

 the whole plant, root, stem, and branch. 



In the whole Monocotyledonous class the first leaves in the seed have 

 the order |. 



In the Dicotyledonous class the first leaves in the seed have the 

 simplest order of the whorled type. 



As the spiral orders have probably been derived from a two-ranked 

 alternate arrangement, so the whorled orders have probably been derived 

 from a two-ranked collateral (two abreast) arrangement. This is illus- 

 trated by an experiment similar to the former ; and it is seen that suc- 

 cessive parallel horizontal pairs of spheres are compelled under contraction 

 to take position at right angles to one another, exactly in the well-known 

 crucial or decussate order. These whorls of two contain potentially 

 whorls of three and four, as is seen in variations of the same plant ; but the 

 experiment does not show the change. 



The reason of the non-survival of the (supposed) two-ranked collateral 

 order lies in its manifest instability ; for under lateral pressure it would 

 assume the alternate, and under vertical the crucial order. 



The bud presents in its shape a state of equilibrium between a force 

 of contraction, a force of constriction, and a force of growth. 



To sum up, we are led to suppose that the original of all existing leaf- 

 orders was a two-ranked arrangement, somewhat irregular, admitting of 

 two regular modifications, the alternate and the collateral ; and that the 

 alternate has given rise to all the spiral orders, and the collateral to all the 

 whorled orders, by means of advantageous condensation in the course of 

 ages. 



VOL. XXI. 



Q 



