AN INTBODUCTION TO PLANT BIOLOGY 117 



We shall later study some microscopic animals which 

 respond to mechanical, heat, light, and electrical stimuli ; 

 but they have no visible nerves or nerve organs. These 

 lower animals have irritability or nervous functions without 

 special organs to perform the functions. This is essentially 

 the case in plants. In recent years there have been many 

 magazine articles discussing " the nerves of plants." The 

 truth is that no one has seen any nerves or brains or similar 

 nervous organs in plants, although they do exhibit the 

 various forms of irritability and respond to the different 

 kinds of stimuli which affect animals, as the following ex- 

 amples will show : 



Mechanical or touch stimuli (such as a jar by passing 

 animals) causes the sudden folding of the leaflets and drooping 

 of the branches of the Mimosa or sensitive plant (see Fig. 1). 

 The leaf of the Venus fly-trap (Fig. 2) closes quickly and 

 grasps insects which happen to touch it. 



Response to light stimulation is seen in the familiar grow- 

 ing of house-plants toward the window, and also in the 

 opening and closing of many flowers. Flowers of the dande- 

 lion and others of the same family, California poppy, etc., 

 open in the light and close when placed in darkness. 



Heat stimulus also causes many flowers to open and close ; 

 tulip, crocus, and " star of Bethlehem " are familiar examples. 

 The combined effect of heat and light stimuli upon the open- 

 ing of flowers is so marked in many species of plants that it 

 is possible to make a flower-clock, which is simply a garden- 

 bed arranged to imitate a clock-dial, in which for each hour 

 of daylight there is a selected group of plants whose flowers 

 are commonly open at that hour. Of course, the flowers do 

 not open exactly on the hour, for the controlling temperature 

 and sunlight vary from day to day. However, many flowers 

 are open in early morning ; certain flowers (example, " ten- 

 o'clock ") open in the middle of the forenoon ; others (like 

 " flower-of-an-hour ") open only in the mid-day sunshine ; 



