322 The Field Naturalist's Quarterly November 



their powers. Localisation of sensation is also to be noticed 

 as important in plants which show movement of any kind. 

 Some have the power of motion in their leaves, others in 

 their flowers ; in others the stamens only can be affected, in 

 others the stigmas alone. All these movements are not 

 merely done for effect, but are of some distinct advantage 

 to the well-being of the plant, either as protection from 

 injurious influences, in the interests of pollination or fertil- 

 isation, or for the food supply. The agents which influence 

 some plants to a large degree are perfectly useless to 

 stimulate others, sometimes even when the same object is to 

 be attained. For instance, the leaf of a Venus' fly-trap will 

 close at the touch of any object, whilst the sundew will not 

 close its tentacles unless nitrogenous material is introduced, 

 both plants requiring nitrogen- — but one showing its hyper- 

 sensitiveness to a chemical stimulus, whilst the other will 

 waste a lot of time and energy in the pursuit of a shadow ! 



The terms sensitiveness and irritability are both used to 

 denote the effects on plants or their special parts from 

 various causes. 



Irritability we may conveniently consider as being the 

 result of mechanical and chemical stimulation, whilst sensi- 

 tiveness may convey the idea of irritability in a modified 

 degree, although there is but little difference between the 

 terms. 



Irritability conveys the idea of sudden or distinct move- 

 ment on the part or parts affected by an outside cause, and 

 sensitiveness the slower realisation of some stimulating cause. 



It is strange that there should be plants which are 

 apparently stolid and unaffected by atmospheric and other 

 conditions, whilst there are others immediately affected by 

 the slightest disturbing element. Some flowers open and 

 close at the approach of light or darkness, others are 

 unmoved. The clover and the wood-sorrel fold up their 

 foliage leaves as night or cloudy weather comes on, whilst 

 the leaves of hundreds of plants around them are quite 

 unchanged. 



There are a few notable examples, familiar to most 

 students, illustrating as well as possible the phenomena of 

 nervous mechanisms. 



