THE ENVIRONMENT 



323 



under minimal conditions that would kill many other 

 species. 



Locally, light intensity can be reduced by many 

 factors. Some of these were mentioned in relation to 

 climate. The more important factors limiting light 

 intensity are thicker air at low altitudes, greater 

 moisture or clouds in the air, solar radiation coming 

 at an angle through the atmosphere (therefore travel- 

 ing through more atmosphere), radiation striking the 

 substrate at an angle, greater depth of water, num- 

 ber and volume of particles in air or water, vegeta- 

 tion cover, and angle and direction of slope of the 

 substrate. Because these factors can cause a variety 

 of light intensities in a locale, unlike responses may 

 be made by organisms living there. 



The duration of light is often called an environ- 

 mental clock because the length of night and day have 

 remarkable associations with plant and animal activi- 

 ties. In plants, those species termed "long-day 

 plants" bloom only after many days with more than 

 twelve hours of sunlight; in "short-day plants" the 

 period of light must be less than twelve hours (ac- 

 tually, the duration of darkness seems most important 

 to these plants). In animals, length of day is as- 

 sociated with reproductive cycles, migrations, molts, 

 and many other life functions. 



PHOTOPERIODISM 



The phenomenon whereby organism responses are 

 related to light duration is called photoperiodism. 

 In plants, photoperiodism is limited to arctic and 

 temperate zone, long- and short-day species. The 

 long-day plants include all species poleward of lati- 

 tude 60° and some in the temperate zone. These 

 plants flower in late spring and early summer, in con- 

 trast to short-day plants which flower in early spring 

 or late summer. The short-day plants are found al- 

 most exclusively in the temperate zone. 



Various plant and animal responses are related 

 to photoperiodism. The more significant plant re- 

 sponses include length of life cycle, flowering, germi- 

 nation, size and structure of vegetative organs, degree 

 of branching, lobing of leaves, pigmentation, forma- 

 tion of storage or fibrous roots, nutrient requirements, 

 dormancy of deciduous plants, and susceptibility to 

 parasitism and disease. Also, photoperiodism is sig- 

 nificant in plant distribution. Its importance is best 

 appreciated in terms of day length, because plants 

 tend to require lengths of day corresponding to the 



locality in which they grow and the season in which 

 they flower. Therefore, the fact that day length shows 

 definite geographic relations causes light duration to 

 be a very important factor in geographic distribution. 

 Neither light intensity nor wavelength are related 

 to plant geography. These two factors display great 

 local variation and are most important as contribu- 

 tors to local peculiarities in the distribution of 

 individual plants and habitats. 



ATMOSPHERE 



The atmospheric gases directly influencing or- 

 ganisms are the 21 per cent oxygen and .03 per cent 

 carbon dioxide. Both plants and animals utilize oxy- 

 gen and release carbon dioxide in the function of 

 respiration. However, plants consume more carbon 

 dioxide than they release and expel more oxygen 

 than they ever utilize in respiration. This condition 

 exists because the daytime process of photosynthesis 

 uses carbon dioxide and expels oxygen to a greater 

 extent than the continuous process of respiration uses 

 oxygen and expels carbon dioxide. Therefore, plants 

 act to prevent potentially poisonous concentrations of 

 carbon dioxide and provide oxygen for most other 

 organisms. 



Atmospheric gases enter and leave plants by open- 

 ings in leaves, by openings in woody twigs, and by 

 diffusion through rootlets (Figure 17.16). Waste gas 

 is expelled only by leaves and twigs. This entire 

 gaseous change is related to transpiration, because 

 all gaseous exchanges necessitate wet cell surfaces 

 and thus cause water loss to the plants. 



Proper plant aeration is essential for normal 

 growth and function, but aeration problems rarely 



-PLANTS 



''photosynthesis 



respiration 



SOIL 



porous 



stagnant 



saturated 



ANIMALS 

 respiration 



AIR 



Figure 1 7.16 Oxygen -carbon dioxide relotionships of plants, ani- 

 mals, and soil with air. 



