THE BIRDS OF 

 SOUTH AMERICA 



Volume II: The Suboscine Passerines 



By Robert Ridgely and Guy Tudor 



This eagerly awaited successor to 

 Volume I, which Audubon called "the 

 major omithQlogical publication of the 

 year," encompasses over 1,000 species. 



940 pages 

 1 ,043 maps 

 52 color plates 

 $85.00 cloth 



Still available 



Volume I: 

 The Oscine 

 Passerines 



$70.00 cloth 



THE BIRDS OF 

 lERICA 



iy Robon S Ridgely 



UNIVERSITY OF TEXAS PRESS 



60x7819 • Austin 78713 

 At bookstores, or call 800-252-3206. 



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lengths in the red end of the spectmm di- 

 minish more quickly than others, the effi- 

 ciency of photosynthesis is reduced. This 

 limits the depth to which plant life can sink 

 before the rate of photosynthesis fails to 

 match the rate at which plants consume 

 the foods they manufacture from sunlight 

 and raw chemicals. This equilibrium, 

 known as the compensation point, varies 

 among plant species, thereby permitting 

 different types of plants to grow at differ- 

 ent depths in oceans and deep lakes. 



Yet another physical law demonstrates 

 that life in freshwater is harder than life on 

 land. Pick's law shows that the rate at 

 which carbon dioxide and oxygen diffuse 

 into cells depends upon a physical prop- 

 erty called the diffusion coefficient. The 

 higher the numerical value of the coeffi- 

 cient, the greater the rate of diffusion. Im- 

 portantly, the diffusion coefficients for car- 

 bon dioxide and oxygen dissolved in water 

 are significantly lower than they are in the 

 air. Thus, all other things being equal, car- 

 bon dioxide and oxygen take a longer time 

 to enter the cells of plants in freshwater 

 than to enter those on land. 



Pick's law and a few rules of elemen- 

 tary geometry also tell us that since gases 

 don't diffuse well into aquatic plants, the 

 best shape for a plant is one that will max- 



imize its surface area in relation to its vol- 

 ume. In other words, to get enough gases 

 for its internal needs, an aquatic plant 

 needs either to remain very small in size or 

 to adopt "high surface area" shapes. Ex- 

 amples are long, cigar-shaped plants or 

 broadly flattened, leaf-shaped plants, such 

 as sea lettuce. 



Finally, all these lessons about the phys- 

 ical properties of water and gases can be 

 used to construct a scenario for the colo- 

 nization of the land by plants. Their small 

 size (dictated by Pick's law) conferred 

 ecological and evolutionary advantages on 

 aquatic plants. Small organisms grow and 

 reach sexual maturity more rapidly than 

 their larger counterparts. Therefore, they 

 can live in ecologically changeable habi- 

 tats. Also, small organisms, with their 

 comparatively rapid life cycles, tend to 

 have higher mutation rates and, as a very 

 general rule, evolve more rapidly than 

 larger organisms. Thus, small plants grow- 

 ing just below the surface of ancient fresh- 

 water lakes or water-saturated soils likely 

 multiplied rapidly and had high mutation 

 rates — features that conferred many ad- 

 vantages when water levels periodically 

 dropped. Only those plants capable of en- 

 during short-term water deprivation and 

 brief exposures to the air could survive 



Horsetail 



0. 



50. 



100. 



150. 



S200. 



>250- 



§300 

 350 

 400 

 450 

 500 

 550-J 



Club Moss 



J£*s^pI 



Grass 



Daisy 



Neogene 



Paleogene 



Cretaceous 



E 



<r^ 



^ «BHHU 



Jurassic 



Triassic 



Permian 



Carboniferous 



Angiosperms 

 (Flowering Plants) 



^J- 



Devonian 



Silurian 



First Vascular Plants 



Ordovician 



Cambrian 



The Great Plant Explosion: The diversity of today's land plants is the 

 resuh of a remarkably rapid period of diversification in the Devonian. 



24 Natural History 6/94 



