221 



The actual value of this motion of the plant may be seen if two 

 metal plates are exposed to the sun, one horizontally, the other verti- 

 cally, iu the middle of the day. A touch of the finger will reveal the 

 difference in temperature an hour later. 



As the sun sinks toward the western horizon the action of its rays 

 decreases and the self regulating leaflets slowly open. The dangers of. 

 the day are by no means over however. As the sun disappears the air 

 grows cooler, and the leaves radiate heat that would result in chilling 

 and other injuries, if allowed to continue. Once more the regulating 

 mechanism is called into pla}' and once more the leaflets are placed in 

 a vertical position when they radiate heat less rapidly. This so-called 

 " sleep position" of leaves may or may not be identical with the noon- 

 day position. 



The metal plates may again be used to demonstrate the physical value 

 of sleep movements. The plate held in a horizontal position for an 

 hour will be found to be cooler than the one held vertically. The test 

 is a delicate one and requires accurate thermometric measurements. The 

 necessity for the movements of the leaflets may be shown if the leaflets 

 of the Guango are fastened in a horizontal position and allowed to 

 remain exposed to the full blaze of the sun and to the night air for a 

 day or two. 



All of the above movements are adaptations by which the plant 

 facilitates the acquisition of food or protects the organs of nutrition. 



It will be profitable to turn our attention for a few minutes to 

 some specialised movements exhibited by some of the common plants 

 of Jamaica. 



The ordinary Shameweed (Mimosa) the pest of your lawns and 

 pastures will serve as an example. The leaflets of this plant beside 

 moving in response to changes in temperature and light, also exhibit 

 reactions to a shock or a b'ow, incision or other injury, electric current, 

 or a flame, and may be made to assume the sleep position by ether or 

 chloroform. Although much attention has been given to this plant 

 yet it is quite impossible to see that any useful purpose is aubserved by 

 movements in response to some of these stimuli. It is safe to say that 

 so far as some of the stimuli are concerned, they are not met with 

 by the plant in its original habitat in Brazil and Venezuela. 



It is to be seen that the Shameweed offers several interesting 

 features which are by no means common. Of these, one of the most 

 striking is the manner in which impulses are transmitted from one 

 branch to another. This may be best illustrated by reference to the 

 diagrams. (See Figs. 3 and 4.) If the flame of a burning match is 



-Fig. 3. Branch of Mimosa showing normal and irritated closed leaflets. 



