66 A PRIMER OF BIOLOGY 



surroundings and of the presence of food. Where 

 response to contact is developed in plants it is, with 

 few exceptions, connected only indirectly with the 

 acquisition of food. More often it is associated with 

 the attempt to obtain support, where the plant is 

 unable by its own unaided efforts to stand erect, and 

 is developed most prominently in tendrils, such as 

 one finds in the pea, Cobaea (Fig. 25), passion-flower, 

 and other climbing plants which possess such organs. 

 In the case of some carnivorous plants, however, 

 response to the stimulus of contact is intimately 

 associated with nutrition as, 

 e.g., in the carnivorous plant 

 Dionsea (Fig. 26), the two 

 halves of whose leaf - blade 

 close immediately on any 

 insect that may happen to 

 touch any one of the six 

 FIG. 26. Dionsea sensitive hairs which arise 



from the upper surface of the 



leaf, or in the case of our own Sundew (Fig. 15), where 

 contact of an insect with the sticky tentacles in the 

 centre of the leaf brings about a slow infolding of all the 

 peripheral tentacles over the trapped animal, and also 

 a rapid secretion of digestive fluid from their glandular 

 ends. In other plants still, such as the sensitive 

 plant (Fig. 27), Mimosa, rapid movement, presumably 

 for protection, takes place in the leaf when touched, 

 and similar movements, due to other stimuli, are well 

 known to occur in other forms, such as Oxalis, 

 Lathyrus, &c. 



A little reflection will show us that the animal 

 on detecting, by contact, an injurious object or an 

 object fit for food, being motile, can at once move 

 away from or towards the object as the case may 



