410 MEMOIRS OF THE AMERICAN ACADEMY. 



fronds of brakes or ferns, is another way of bringing the moving medium to 



impinge on the leaf-surface ; but the feasibility of this depends on the fibrous 



framework which the leaves of land-plants have acquired for the support of 



their softer tissues. Such a segmentation also appears among the higher 



plants in compound leaves and in whorls ; and, indeed, the whole foliage of 



trees and shrubs may, from this point of view, be regarded as the reduced 



segments of the blades of branching fronds, turned in all directions in search 



of light, and inviting the movements of air through their expanded interstices. 



Such is the kind of utility that may be claimed for the structure of our 



hypothetical spiral frond. Another utility in this structure is obvious when we 



consider the transition of plant-life from aquatic conditions to those of the dry 



land and the air; as vegetation slowly crept from its watery cradle, or was left 



stranded by the retiring sea. In default of strength in its material, such as a 



acquired fibrous structure or framework ultimately gave to it in this 



transition, the strongest form would be the most advantageous in sustaining 



the weight of the no longer buoyant plant. A spiral arrangement of the blade 



around a comparatively firm, and, perhaps, already somewhat fibrous stem, 



would come nearer fulfilling this condition than any other conceivable modifi- 

 cation of the frond. 



We have, so far, in conformity to the spiral arrangement in leaves, supposed 

 this twisted frond to be a single-bladed one, or with only one blade developed. 

 This would be a first step in that reduction of leaf-expansion which a more 

 advantageous situation of it would allow; and might be required, even at 

 this early stage of atmospheric plant-life, on account of the greatly increased 

 importance of the roots and stem. But this hypothesis is not necessary in gen- 

 eral for ^the ends we have considered. A two-bladed frond might be similarly 

 twisted and give rise to a double spiral surface like a double spiral stairway, 

 or like the blade of an auger; or such a surface as the two handles of the 



auger describe as they are revolved, and, at the same 



ed forwards 



the direction of the boring. The simplest segmentation of such a twisted frond, 



the stem had acquired sufficient strength, and such a subsequent reduction 



after 



of the segments as might be required for the nutrition of the stem, would give 

 rise to parts, which, turned upwards to face the sky, and also separated, perhaps, 

 by the growth of internodes in the lengthening stem, would result in what we 



O """"""& 



may regard as the original form of whorls, namely, a continuous leaf-like expan- 

 sion around the stem. The origin of the whorl arrangement itself would thus 



