SENSATION 



digestion, and corresponds to the gastric juice in the 

 animal, is only secreted by the corpuscles if the solid 

 foreign body is nitrogenous (flesh or cheese). Hence 

 the leaves of these insectivorous plants taste their meat 

 diet, and distinguish it from other solids, to which they 

 are indifferent. In the broader sense, in fact, we may 

 describe the points of the roots of plants as organs of 

 taste; they plunge into the richer parts of the earth 

 which yield more nourishment, and avoid the poor parts. 

 In unicellular plants and animals the action of chemical 

 stimuli is especially conspicuous when it is one-sided, 

 and provokes definite movements in one particular 

 direction (chcmotaxis). 



The movements of unicellular organisms that are 

 provoked by chemical stimuli and are known as chemo- 

 tropism (more recently as chcmotaxis) are particularly 

 interesting because they show the existence of a chemical 

 sensitiveness, somewhat resembling taste or smell, in the 

 lowest organisms, and even in the homogeneous plasm 

 of the monera. Repeated experiments of Wilhelm 

 Engelmann, Max Verworn, and others, have shown that 

 many bacteria, diatomes, infusoria, rhizopods, and other 

 protists, have a similar sense of taste; they move 

 towards certain acids (for instance, a drop of malic acid) 

 or a bubble of oxygen that lies on one side of the drop of 

 water in which the protists are under the microscope. 

 Many pathogenetic bacteria secrete poisonous sub- 

 stances which are very injurious to the human frame. 

 The active white blood-cells, leucocytes, in the human 

 blood have a special "taste" for these bacteria-poisons, 

 and concentrate in large quantities, by means of their 

 amoeboid movements, at those parts of the body where 

 they are secreted. If the leucocytes prove the stronger 

 in their struggle with the bacteria, they destroy them, 

 and in this way they act as sanitary officers in keeping 

 poisonous infection out of our organism. But if the 

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