Aquatic Plants 235 



dividually and a continuous canopy is maintained, with the result 

 that little vegetation can grow on the forest floor. 



In such rainforests many smaller plants have gained access to suf- 

 ficient illumination by the evolution of the epiphytic habit, that is, 

 by growing in the crotches or on the horizontal branches of the trees. 

 The vertical distribution of the many species of bromeliads in the 

 forest trees of Trinidad reveals their varying degrees of tolerance 

 (Fig. 6.27). One group of species grows only near the tree tops in 

 situations fully exposed to the sun, a second group is found at inter- 

 mediate levels in partial sunlight, and a shade-tolerant third group 

 inhabits the lower branches of the forest where the illumination is 

 quite inadequate for the other groups. 



Aquatic Plants. Reduction of light presents even more serious 

 problems in the aquatic environment. As we have seen illumination 

 diminishes rapidly with depth even in clear water and becomes 

 changed in spectral composition and in other respects. Plants at- 

 tached to the bottom in the marine environment consist principally of 

 algae with a few species of vascular plants such as the eel grass. In 

 fresh water vascular plants as well as algae are well represented in 

 the submerged vegetation. 



Early investigators reported an apparent color zonation in the depth 

 of occurrence of attached plants. In Puget Sound, for example, the 

 green algae are generally the most abundant in shallow water, brown 

 algae dominate the zone from 5 to 20 m, and red algae are usually 

 most numerous in depths of 10 to 30 m. These color types of algae 

 often occur at depths where the complementary color of the penetrat- 

 ing daylight predominates— red algae, for example, tend to be abun- 

 dant in deep water where the blue or green component of daylight is 

 the strongest. It was formerly believed that the predominating color 

 of the light controlled the depth distribution of the color types of 

 algae because the plants would absorb light of complementary color 

 more efficiently. This generality was referred to as cliwrnatic adap- 

 tation. Many exceptions occur, however, and we now know that the 

 wavelength of light does not control in any precise way the depth of 

 distribution of algae on the basis of their color (Sutton and Juday, 

 1944). Furthermore, the pigment of algae living in weak light deep 

 in the water is sufficiently thick to absorb all the light incident upon it, 

 regardless of the part of the spectrum in which it occurs. Evidence 

 exists that some of the light energy adsorbed by pigments other than 

 chlorophyll can be transferred to the chlorophyll present and thus 

 enhance the ability of certain algae to live in weak blue-green hght 

 (Smith, 1951, Ch. 13). 



