36 SOME PROPERTIES OF CELLS 



Some of the most important stimuli which call out manifes- 

 tations of irritability in protoplasm are heat, light, electricity, 

 gravity, pressure of external objects, and contact with substances 

 which act chemically on the protoplasm. Many instances of 

 irritability will come up in later chapters. A notable example 

 of response to a stimulus is the beginning of germination in 

 seeds subjected to a suitable degree of heat in presence of 

 moisture. 



The ways in which the responses to stimulation may show 

 themselves are very numerous, and the same individual or organ 

 may be favorably affected by a certain amount of a given stimu- 

 lus and unfavorably by a greater amount of the same stimulus. 

 Every one has had the experience of drawing near to a moder- 

 ately heated stove in cold weather and then retreating from it 

 when the fire grew too hot. So, too, certain microscopic uni- 

 cellular plants, living in water, move toward the light until it 

 reaches a certain intensity, but when that intensity is passed, 

 they move in the opposite direction, toward the dark. 



47. Selective absorption. Another extremely important power 

 of live protoplasm is that of selective absorption. By this is 

 meant the ability to take up from liquids or gases certain sub- 

 stances and leave unabsorbed other elements or compounds 

 which are also present. 



Thus plants of two different species, both growing in the 

 same soil, usually take from it very various amounts or kinds 

 of mineral matter. For instance, barley plants in flower and 

 red clover plants in flower contain about the same proportion 

 of mineral matter (left as ashes after burning). But the clover 

 contains 5| times as much lime as the barley, and the latter 

 contains about 18 times as much silica as the clover. This dif- 

 ference must be due to the selective action of the protoplasm 

 in the absorbing cells of the roots. 



48. Osmosis. The process by which two liquids of different 

 densities separated by membranes pass through the latter and 

 mingle, as soil water does with the liquid contents of root hairs, 



