302 Dr. H. Airy on Leaf -Arrangement. [Apr. 30, 



vertical, to a form in which they are strongly twisted into a com- 



O (y 



plex order with angular divergence nearly -?, differing from | by only 



7 o 



- of the circumference, and evidently admitting of further twist and 

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closer approximation to the order |. Prom this striking series it is 



inferred that ranks originally vertical can and do acquire and transmit a 

 tendency to deviate from the vertical, and that this tendency admits of 

 augmentation to a high degree. 



Assuming a twist, then, as a probable primary variation from an 

 originally vertical condition of leaf-ranks, it is plain that each leaf would 

 take a lower position, and the whole bud (with the same number of 

 leaves) would be shorter, than in the untwisted form. The shorter bud, 

 it is supposed, would have an advantage in cold seasons. The direct 

 action of cold, by stunting the bud-axis (provided it did not stunt the 

 leaves in the same proportion), would increase the twist. It may fairly 

 be supposed that this twist would be taken advantage of and increased 

 by natural selection in subservience to the close packing of the leaves. 

 This course of modification is equivalent to the continued action of a 

 force of vertical compression (mentioned above as the second requisite for 

 condensation). 



Transition similar to that in Gasteria is seen in the genus Aloe. 

 Compare the two vertical ranks of A. verrucosa with the two twisted ranks 

 of A. obliqua. In A. serra (Sachs, ' Lehrbuch der Botanik,' fig. 144) 

 the change from the vertical to the strongly twisted form is found in the 

 same plant : the basal leaves are in order ^ ; the higher take complex 

 order. 



Exactly comparable (in this respect) with Aloe serra are the common 

 laurel, Portugal laurel, Spanish chestnut, ivy, and others, which exhibit a 

 similar change of leaf-order. These instances agree in presenting the 

 complex order in the buds or parts of buds which occupy the most exposed 

 situations, while they retain the simple order ^ in the less exposed 



lateral buds or in their basal portion. The exposure in the former 

 case may be regarded as a sample of that which, in the course of 

 many generations, has (it is supposed) occasioned the condensation of 

 leaf -order. 



It is here contended that the force of gravity (to which the two- 

 ranked leaf-order of lateral twigs is referred by some authors) couM 

 not have been equally the cause of the phenomena seen in the inclined 

 lateral shoot of Spanish chestnut and in the upright Aloe serra: but the 

 phenomena in the two cases are the same, and admit of a common 

 explanation by the condensation theory, if we regard the basal portion 



