saturated, so that water docs not quickly evaporate from the surfaces, ab- 

 sorption from the soil may be stopped. 



If soil minerals are present in certain proportions, or concentrations, the 

 plant absorbs accordingly. But if there is too little, then the plant absorbs 

 and discharges more gallons of water for every grain of salt. Or if there is too 

 much salt, the flow through the root cells is outward instead of inward. Plants 

 living in salt marshes are in many ways like desert plants, absorbing water 

 against great resistance, or drying up! 



With changes in temperature, most plants continue their metabolic activi- 

 ties more rapidly or more slowly. But sudden or extreme changes stop metabo- 

 lism. And temperature affects also evaporation, or transpiration. Changes in 

 illumination also alter the metabolism, especially photosynthesis, ^nd the 

 rates of growth of the various parts. / 



Such variations in conditions influence plants, but they do not, as a rule, 

 bring out any striking reactions. Plants seem obliged to take what happens 

 as it comes, since they are not able to run away, or dodge, or hit back. Here, 

 then, the "struggle" is between a particular organism and changes in the 

 surroundings. The particular organism, which seems to us rather passive, 

 does not really remain as it is very long. A plant does move, if less slowly 

 than most animals. It responds to stimulation or to changing conditions by 

 moving — ^so slowly in most cases that we have to take special pains to see 

 what happens. 



Plants Are Sensitive and Active^ The simplest evidence that plants are 

 more or less sensitive we may see in the destruction that results from some 

 external change. A plant may be poisoned or overheated or chilled. If the 

 changes are not too severe, however, the plant behaves in ways that, on the 

 whole, protect it from injury. Tropisms (see page 256) on the whole prevent 

 injury, or they increase the likelihood of getting needed supplies. Some 

 plants can capture animal food, in the form of small insects (see illustration, 

 p. 542). Some reduce the exposed surfaces when disturbed by too much sun- 

 shine, as the eucalyptus tree. Some close down in the dark, as the clo\'er or 

 sorrel. And very many drop their leaves in the autumn, apparently in re- 

 sponse to a shortage of water. 



Generally speaking, however, plants respond to external changes very 

 mildly compared with familiar animals. The success of the indi\idual plant 

 in living through a season of changes seems to depend very largely upon the 

 structures and qualities that it develops from the time it starts out as a sprout- 

 ing seed. Continuing to live depends upon the kind of skin and bark or 

 spines that it grows, or upon the kind of conducting and mechanical tissues it 

 develops, or upon the delicacy and efficiency of its food-making equipment 

 and its food-storing mechanism. And the success of the species depends upon 



iSee No. 2, p. 558. 

 541 



