506 THE COMMUNITY 



trum. The intermediate green band is photosynthesizing protoplasm engaged in 

 largely reflected, causing the leaf to be the common ecological response of organ- 

 green to our eyes. isms and communities to the physical en- 

 The chief adaptation of terrestrial plants vironment, in the synthesis of carbohydrates, 

 for carrying on photosynthesis is the leaf, This large-scale industry is absent from 

 and leaves work most efficiently when at few places, though possibly from certain 

 right angles to the fight beam. This posi- areas of waterless deserts and from iso- 

 tional arrangement is well shown in forests, lated mountain peaks. This brings up the 

 where, from canopy down to herbaceous question, raised previously for phytoplank- 

 stratum, the response to fight exercises a ton, of the efficiency of this photosynthesis, 

 profound effect upon stratification in the This logical extension of the argument re- 

 large and in the individual response of quires much research by physiologists. Its 

 each leaf. Several pertinent examples are answer is relevant to community metabo- 

 discussed by Thimann (1941), such as the fism as well. 



maple sapfing, which has each leaf at right Fortunately, Transeau (1926) has given 



angles to the incident fight. In the com- us one estimate, based upon a sun-tolerant 



Table 43. Photosynthetic Efficiency of Field Corn (After Transeau, 1926) 



Total dry weight of an acre of corn (10,000 plants) 6000 kg. 



Less ash (inorganic matter) 300 kg. 



Total organic matter 5700 kg. 



Equivalent of this in glucose 6700 kg. 



Plus organic substances lost by transpiration for the season, 



expressed as glucose 2000 kg. 



Total glucose formed by an acre of corn 8700 kg. 



Energy required to synthesize 1 kilogram of glucose 3800 KCal. 



Energy required to synthesize 8700 kilograms about 33,000,000 KCal. 



Total solar energy available for one acre 2,040,000,000 KCal. 



Therefore: % available energy used = x 100 = 1.6% 



2040 million 



pass plant of the Ilfinois prairie, this leaf species, field com. His calculations appear 



adjustment is supplemented by rotation and in Table 43. 



curvature of the leaf stalk. Other factors. Plankton sampfing, board feet of lumber, 



especially sufficient moisture, will allow "a or tons of hay, per unit of area-time, are 



very large fraction of the land surface to ^^^^^ ^^ i^^^^j o^. ^^^ ^e obtained or esti- 



be covered with green leaves" (Thimann, ^^^^^ -j^j^^ essential energy relationships 



.,'.,?■ 1 . , T 1 . are usually not available and are difficult 



Abihty to bnng leaves perpendicular to ii f 



incident fight and to curve fightwards . ' . ,.r, , . r i 



through hormone regulation, results in an ^ oversimpfified view of energy rela- 



ecologically advantageous position for each tionships can be obtained by a considera- 



plant. Thus each fixed forest plant makes tion of photosynthetic productivity in terms 



the most of its total leaf surface. From a of amount of glucose produced per unit of 



synecological aspect, this results in maxi- area and time for several "average" com- 



mal photosynthesis for the whole commu- munity types. Such a comparison is pre- 



nity and is equivalent to vertical movement sented in Table 44, and will prove inter- 



of plankton populations as previously esting in a discussion of total community 



noted. metabolism, if the hypothetical nature of 



It means much more than this. If we some of the conclusions is remembered, 

 combine the generafizations of Nielsen As we have seen (p. 502), the plank- 



(1934) and Thimann, there emerges a tonic photosynthetic industry of aquatic 



much larger, global one, namely that a communities is characterized by seasonal 



large part of the planet's surface is covered rhythmicities of its component populations, 



with a relatively thin, taxonomically com- The major seasonal rhythm in deep lakes 



plex, structurally discontinuous layer of includes a high vernal, and a much lower 



