262 THE LIFE OF THE PLANT 



of the dandelion which spoil so often the uniform green 

 of our lawns. This is because these flowers open only 

 in the light : on a dull day they may likewise remain 

 closed. Other flowers, on the contrary, close in the 

 day-time. This is the case with the goat's -beard 

 (John-go-to-bed-at-noon), the yellow flowers of which are 

 very much like those of the dandelion, only larger in size. 

 They open early in the morning and close by ten or 

 eleven. These phenomena attracted much attention 

 among botanists of the eighteenth century. It was even 

 suggested that floral hours might be observed ; the 

 hours of the day being defined by the opening and closing 

 of different flowers. 



These phenomena are easily proved to depend on the 

 action of light and heat. The crocus is especially con- 

 venient for the purpose. Its large flower opens in the 

 day-time and closes at night ; but the same phenomenon 

 can be caused by placing it alternately in the light 

 and in the shade, or removing it from a warm place to 

 a cold one, and vice versa. A difference of temperature 

 of ten or twenty degrees makes it close and open again 

 in a few minutes. We can explain these phenomena 

 by unequal growth or by the tension of tissues in 

 the upper and lower, or outer and inner layers of the 

 moving organ. We have already seen that light 

 retards growth ; consequently, under its influence, the 

 growth of the outer layers will be checked while the 

 inner layers will outgrow them, and the organ as a whole 

 will tend to curve outwards. As a result the flower will 

 open. Now it is the inner (or upper) side that becomes 

 exposed to greater illumination, while the outer (or 

 lower) side, being shaded, will outgrow it and the flower 

 will close. Similar reasoning can be applied to the 

 effect of changes in temperature. 



Such is the nature of these phenomena. They can 

 ultimately be attributed to irregular growth, and, as a 

 matter of fact, they are generally observed in organs 



