116 APPLIED BIOLOGY 



nitrogenous excretions (those containing nitrogen) formed in 

 animals. While the plant cannot eliminate these substances 

 directly, it is interesting to note that many of them, along 

 with mineral substances taken up with water from the soil, 

 become stored in leaves, fruit, seeds, and bark all of 

 which parts of plants are frequently detached, resulting 

 in the elimination from the plants of useless and possibly 

 sometimes harmful substances. In many other cases sub- 

 stances of no further use to the plant may be stored per- 

 manently in harmless forms in stem or roots. 



This habit of storing certain useless substances is peculiar 

 to plants. As plants grow older the amount of stored sub- 

 stance, especially mineral matter from the soil, increases. 

 Thus a leaf collected and burned in early summer leaves 

 little ash as compared with a similar one taken in late autumn. 

 Throughout the growing season water has been evaporating 

 and leaving behind in the leaves the mineral materials carried 

 up from the soil. 



Animals do not store excess or useless mineral substances 

 and excretions, but eliminate them daily, dissolved in 

 water which is discharged by means of the kidney-system. 

 Since most plants cannot discharge water in liquid form, 

 storage of useless mineral matter and some excretions is 

 the plant's only possible way of doing the same kind of 

 necessary work which the kidneys of animals do. 



108. Irritability of Plants. Irritability in either plants or 

 animals is the power of responding to a stimulus. For ex- 

 ample, if a frog be touched suddenly (mechanical stimulus), 

 the leg muscles contract and the animal jumps. The frog also 

 responds to heat stimulus (goes into shade when the sun's 

 heat is too great) ; to light stimulus (sees enemies and jumps) ; 

 to sound stimulus (hears sounds and jumps) ; electrical 

 stimulus (jumps if touched by a slightly charged electric 

 wire). These are phases or kinds of irritability common in 

 animals which have a brain, spinal cord, and nerves. 



