How Plants Resist Hostile Forces Around Them 259 



thought is enough to confirm it; for, although morphologic- 

 ally single leaves, they are cleft to a great many small leaflets, 

 each of which acts physiologically as a single leaf. This division, 

 or ''compounding" (as it is called scientifically), of leaves in such 

 plants appears clearly to constitute a protective adaptation 

 against the tearing action of winds; and I believe the same factor 

 is the principal one in determining the compounding of leaves in 

 general, though sometmies the compound condition, as in our 

 undergrowth Ferns, means rather a persistence of an ancestral 

 condition than anything of immediate importance. Nor is one's 

 natural thought at this point, that the sizes of leaves are de- 

 pendent on their thickness, correct. The thickness of leaves is 

 determined by the depth to which sunlight can penetrate green 

 tissues without losing all of its photosynthetic power; and hence 

 it is approximately the same in all leaves exposed to the sun in 

 the same climate, with a trend towards more thickness in extra- 

 bright places, and thinness in shade. Undoubtedly the whole 

 tendency of wind action is to produce an adaptive lessening in 

 size, which is directly antagonistic to the tendency of photo- 

 synthesis to produce a larger spread of surface; and the resultant 

 between the action of these two factors, modified it is true by 

 certain other minor influences, makes leaves the sizes they are. 

 This explains why our common deciduous trees of similar habit, 

 our Oaks, Elms, Maples, and Chestnuts, possess leaves of much 

 the same size, or at least of the same order of magnitude. That 

 size represents the equilibrium between the contesting photo- 

 synthetic and wind factors acting on leaves of standard thick- 

 ness growing in similar situations. 



Another kind of strain to which plants are exposed is the 

 weight of the winter's snow and ice. This danger is greater, of 

 course, for evergreen than deciduous trees, but against it the 

 conical shape characteristic of evergreens provides a manifest 

 protection. This follows from the fact that only the ends of the 

 branches are exposed to the falling load, while their slender forms 



