THE BUILDING-STONES OF THE ORGANIC WORLD 65 



organic food. Numerous other parasitic growths have no 

 chlorophyll, but are nevertheless true plants. 



Since the time of Linnaeus the faculties of nutrition and 

 reproduction have been generally ascribed to plants, whilst 

 animals were said to possess in addition the faculties of sensation 

 and motion. These distinctions, however, can no longer be 

 maintained. , -, 



We have already seen that numerous lower animals sponges, 

 corals, and sea-anemones possess no power of locomotion. 

 Sponges do not even exhibit any externally noticeable signs of 

 motion, and are all but insensible to every kind of stimulus. 

 Even among the higher animals many have given up their free 

 life ; various species of crabs, for instance, Acorn-shells and Bar- 

 nacles, as well as numerous tunicates which for various reasons 

 are regarded as the precursors of the vertebrates, are fixed to the 

 ground. On the other hand, there are plants which in response 

 to certain stimuli are able to execute distinct vivid movements. 



In hot-houses we often meet with the famous sensitive plant 

 (Mimosa pudica), known in tropical countries as a most objec- 

 tionable weed. The leaves of this plant are 'beautifully divided, 

 again and again pinnate, with a great number of small leaflets 

 of which the pairs close upwards when touched. If we shake 

 the plant, the leaflets close together, the pinnae sink down, then 

 the leaf-stalks sink down, and the whole leaf hangs as if 

 withered.' For this reason popular fancy has called the mimosa 

 the ' chaste flower/ 



Striking and apparently voluntary movements are executed 

 by various ' carnivorous ' plants of which we know at present 

 about five hundred species. The ability to react to a stimulus 

 has in some of these plants reached a very high degree. Darwin's 

 investigation showed that in the well-known Sundew (Drosera 

 rotimdifolia) a curving of the glandular ' hairs ' took place in 

 response to a stimulus exerted by a particle of hair ^ mm. 

 in length, or of 30 ,o 00 nig. of phosphate of ammonia. It is a 

 remarkable fact that it is impossible to deceive these ' tentacles ' 

 and entice them to useless exertions, for the movements are 

 only executed if the leaves are touched with a nitrogenous body, 

 that is, one that may be used for food ; when touched with non- 

 nitrogenous minerals no reaction takes place. 



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