ECOLOGICAL i6i 



two of the outer tentacles, but it becomes more copious, and the 

 stimulus passes to adjacent parts. There is no doubt as to the 

 transmission of the stimulus; and in some species, e.g. Drosera 

 longifolia, several leaves may be involved in the capture. But how 

 this is brought about is unknown; it may be that a large insect 

 stimulates several leaves in its struggle. 



The experiment has been made of stroking the head of one of 

 the central tentacles with a finely pointed match, the result being 

 that in a few minutes the marginal tentacles begin to move inwards. 

 When the head of a single marginal tentacle is strongly stimulated, 

 it bends in to the short central tentacles, and only then do its 

 neighbours at the margin follow suit. It looks as if the stimulus is 

 conducted outwards, but not inwards, in the leaf-blade. This is 

 curiously like a reflex action in an animal, but there are no nerves 

 or muscles in the plant — only irritable and mobile protoplasm. Yet 

 another resemblance between the sundew and an animal is that 

 quite different stimuli may evoke exactly the same response. Just 

 as a muscle may contract under mechanical, nervous, or electrical 

 stimulus, so the sundew's tentacle may move when touched by 

 a solid body (which produces rapid differential compression, in 

 neighbouring areas of the cell-substance) or by certain nitrogenous 

 substances in solution. 



As to the movement which occurs at the base of the tentacle, 

 it is sometimes ranked as analogous to those of growth. This view 

 deserves to be stated. "The basal part is able to renew its youth and 

 start growing again when excited. The growth is unequal and is 

 mainly or wholly confined to the outer or lower side. An increase 

 in length of that side, relative to the upper side, means that the 

 tentacle bends inwards. The straightening or bending outwards 

 takes place by the opposite process of a relative increase in the 

 length of the upper side." (Macgregor Skene, Common Plants, 1921, 

 p. 54.) It follows, one would think, that the movements of a full- 

 grown tentacle must be very infrequent, else it would become too 

 long! Moreover, the movements referable to differential growth- 

 rates are much slower than in the sundew. Another view is that the 

 movement is due to varying flaccidity in the tissues at the base of 

 the tentacle. For if the turgidity of the inner cells at the base of the 

 tentacle is relaxed by the excitation due to the stimulus, there is 

 no need to invoke growth in the outer cells. This is, of course, an 

 intricate question, in regard to which we may profitably agree to 

 differ for a while. After elaborate study Bose concludes that a 

 nervous-like communication from the point of stimulus alters the 

 turgidity of the cells of a motor organ. 



The question remains as to the advantage of the insectivorous 

 habit to the sundew. As its leaves contain chlorophyll, partly 

 disguised by the red cell-sap, the sundew can make carbon-com- 



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