The Profound Effect on the Structure of Plants 65 



the lower fractions of the spiral system, where one can see the 

 advantage of a spacing which may give to the leaves the best 

 aggregate exposure to light. But this interpretation meets in- 

 creasing difficulties with the higher fractions, and even has trouble 

 with the lower when one notices how freely the leaf-blades, the 

 very parts which need the exposure to light, are swung by their 

 slender petioles into positions of advantageous individual exposure 

 in callous disregard of the orderly arrangement in which they start 

 from the stem. There is, however, another and very different 

 explanation of the systems of phyllotaxy advanced by some in- 

 vestigators, viz., that they are wholly determined by the positions 

 in which the young leaves originate inside of the growing bud, 

 which positions in turn are determined by mechanical principles 

 connected with the easiest mode of origin of new swelling parts 

 in buds of a certain size and shape. In other words the fractions 

 of phyllotaxy are merely an incidental result of mechanical 

 conditions present in growing buds, and have only a secondary, 

 if any, reference to adaptation. This explanation I believe to be 

 substantially correct. It is of course not an explanation of 

 phyllotaxy, but merely a transference of the problem into an- 

 other field, as most of our explanations are. But I dwell upon the 

 subject at this length because phyllotaxy seems to me to offer a 

 fairly clear case in which a conspicuous feature of plant structure 

 has merely an incidental and not an adaptive origin. 



There is one other feature of leaf and stem structure to which I 

 have not yet made any particular reference, and that concerns 

 their sizes, which are wonderfully diverse in different plants. 

 Leaves are measured in terms of feet in Bananas and Palms, but 

 need the assistance of lenses to show them at all in some of the 

 kinds that grow in the deserts; they are merely of tissue thinness 

 in some kinds of Ferns, but cyUndrically-thick and stem-like in 

 Aloes and Century Plants. Stems display a thousand feet of 

 length in the Rattan Palm, but are invisible supports to tufts 

 of leaves in the Houseleek; nearly as thin as a hair in some Ferns, 



