SOME MOOT POINTS IN NATURAL HISTORY. 263 



exercising sensation, or for acquiring a certain knowledge 

 of its surroundings, in the lowest grades the nervous power 

 is but ill-defined, and partakes rather of the character of a 

 low and diffused sensitiveness than of definite perceptions. 

 An animalcule such as the amoeba, whose body consists of 

 a speck of structureless protoplasm (Fig. 41), will push out 

 its soft substance to encompass the particle of nutriment 

 against which it has stumbled, as it were ; and although the 

 primitive simplicity of its body forbids us to expect that we 

 should descry definite nerve-structures or any of the other 

 organs which belong to higher states of existence, we may 

 not doubt that the being still possesses diffuse sensitiveness 

 of a low kind. Whilst the fact that a particle of food is 

 seized and engulfed amid its substance, argues strongly in 

 favour of the idea that this endowment of sensitiveness is 

 intended to serve a well-defined end, namely, that of aiding 

 in the nourishment of the organism. Recognising then, the 

 general presence of sensation in animals, and that this 

 sensitiveness serves as a means for acquainting the animal 

 with its surroundings, and for guiding the acts of its life, we 

 may next inquire if sensation is unrepresented in any of its 

 aspects in the world of plant life ? 



The answer to this question may be fitly prefaced by 

 the brief recital of some of the more typical cases of so- 

 called sensitiveness or irritability of plants. With the 

 common wood-sorrel (Oxalis acelosella) almost every one is 

 well acquainted. The leaves of this plant exhibit a three- 

 fold or ternate arrangement, each leaf bearing three leaflets 

 at the extremity of the elongated leaf-stalk. If the leaflets 

 be observed at any period before midday, they are seen to 

 lie flat and horizontally, the edges being in contact. But 

 if during the heat of the day the leaf-stalk be tapped smartly, 

 each leaflet may be seen to gradually fold upon itself, and 

 the leaves, ultimately come to depend in a loose fashion 

 from the stem. When also, the sunlight has ceased to play 

 upon the plant, the same act of closing its leaves is to be 

 observed. Of a much better defined character, is the 



