WHAT IS A PLANT? 159 



can only select a few examples. We will consider them more in 

 detail in a future paper on the " Power of Movement in Plants," 

 when we shall classify these movements according to their respec- 

 tive stimuli. 



Light is in many cases the acting stimulus. This is seen 

 notably in Desinodiiim^ the " Telegraph-Plant," where the large 

 terminal leaflet descends as the sun falls, the two lateral leaflets 

 meanwhile perpetually moving up and down ; in the Compass- 

 pla7it of American prairies, which invariably grows with its leaf- 

 edges N. and S., while their surfaces are placed E. and W., a pro- 

 perty made use of by the trappers on a dark night, in order to 

 find out in what direction they are going ; in the stem of Hdian- 

 thus, the Sun-flower; in the southern inclination of ripening ears 

 of corn ; and in many other cases. 



Similarly, we have the opening and closing of many flowers at 

 certain hours of the day. Heat operates as a stimulus here, as 

 well as light. Bindweed opens at 3 a.m., Chicory at 5 a.m., 

 Pimpernel at 8 a.m., Evening Primrose at 6 p.m., Evening 

 Campion at 7 p.m. ; Goatsbeard closes at noon, and so is called 

 " John-go-to-bed-at-noon " by the dwellers on the Wye; Pimpernel 

 at 3 p.m., Daisy at sunset. Thus Linnaeus made his "floral 

 clock" ! Then, again, the varying periods of the year (these 

 further varying with geographical position) at which flowers open 

 have to do with the stimuli of light and heat. 



Moisture is a stimulus to plant-movement. This is seen in 

 many " hygroscopic " plants. Bindweed and Pimpernel close on 

 the approach of rain, the latter thus called the " Poor Man's 

 Weather-Glass." So, too, the "elaters" oi Equisetiun uncoil in 

 damp weather, and help to scatter the spores ; a similar cause is at 

 work with the " annulus " surrounding the spore-cases of some 

 Ferns, enabling it to contract, and set free the spores. 



Something closely akin to all these is seen in the " sleep " of 

 leaves, such as those of Wood-sorrel, Balsam, Melilot, and notably 

 of Mimosa^ the Sensitive plant. 



Mechanical irritation is the only remaining stimulus we can 

 notice. A footfall will cause the leaves of the Wood-sorrel to 

 close ; a touch, the vibration of a passing train, or even of a 

 trotting horse, will similarly affect Mimosa^ an insect, as it usually 



