THE PHYSICAL ENVIRONMENT OF ORGANISMS 



553 



duction, the dependence upon permanent or recurrent water films in the 

 terrestrial habitat (zygote formation in the mosses and ferns), or the 

 development of special terrestrial adaptations. Among animals, the latter 

 have been chiefly the habit of internal fertilization and the development 

 of either shelled eggs or a viviparous habit. In addition, the terrestrial 

 vertebrates have acquired special extraembryonic membranes (allantois 

 and amnion) and the amniotic fluid, as reproductive adaptations for 

 terrestrial existence. In plants ter- 

 restrial adaptations for reproduc- 

 tion have been associated with the 

 emphasis placed upon the sporo- 

 phyte generation and involve the 

 development of spores or pollen 

 grains that are passively carried by 

 wind, water, or insects. 



The Physical Factors That Affect 

 Organisms 



In whatever medium or media the 

 organism lives, it is subjected to a 

 number of physical conditions and 

 energies that not only modify and 

 condition the medium but also 

 govern and limit the activities of the 

 organism. Among these factors are 

 temperature, quantity and intensity 

 of light, humidity, currents in the 

 medium, the pull of gravity, the 

 pressure of the medium, and the 

 presence and concentrations of 

 various chemical substances. 



Temperature. Measured temperatures within our universe range 

 from close to absolute zero ( — 263°C.) to more than 6000°C, the 

 approximate surface temperature of the sun. Active life processes, how- 

 ever, are limited to a range between a few degrees below 0°C. and some 

 60 or 70° above. Quiescent states, in which an organism may survive for a 

 time with a nearly complete cessation of metabolic processes, would 

 extend these limits perhaps a score or more degrees in each direction. 

 For any one kind of organism, the limits of toleration for temperature 

 extremes are narrower, usually very much narrower, than those indicated 

 above, and somewhere within its own specific limits of toleration each 

 organism has an optimum temperature most advantageous for its needs. 



Fig. 32.9. A small fresh-water food stinger. 

 Hydra extends its tentacles and waves them 

 gently through the water. Small plankton 

 organisms that touch them are stung by the 

 poisonous nematocysts imbedded in some 

 of the ectodermal cells, and are held to the 

 tentacles by the nematocyst threads while 

 the arms are contracting and the prey is 

 being pushed into the mouth. (Courtesy 

 General Biological Supply House, Inc.) 



