SENSITIVENESS AND IKRITABILITY OF PLANT ORGANS. 157 



irregularities of the surface of the particles. An excretion 

 of mucilage appears to follow, which fixes the organ to the 

 foreign support. The irritation not only affects the epider- 

 mal layer, but the subjacent tissues as well, which then assist 

 the former in grasping the support.* 



Another result of growth due to external agencies is seen 

 in the hypertrophied stipules of Acacia sphcerocephala and 

 the stems of Myrmecodium, etc., in consequence of the irritation 

 set up by ants. Dr. Beccari f (and M. Treub J) has examined 

 these " Ant-plants," which occur in Uuhiacece, Mijristicacece, 

 Eupliorhiacece, Verhenacece, Melastomacece, and Palmce, and 

 explains the abnormal structures by variability and heredity. 

 A small swelling appears on the tigellum of Myrmecodium 

 serving the purpose of a reservoir of water, but which only 

 grows larger through the agency of ants. These creatures 

 induce hypertrophy of the cellular tissue. This, then, be- 

 comes hereditary. I would venture to go further, and 

 attribute the large honey-pits at the base of the leaf-stalk on 

 Acacia sphcerocephala, as well as the terminal " fruit-bodies " 

 occurring on the tips of the leaflets, to the same cause, viz. 

 the mechanical irritation of the ants. 



There is, in fact, an abundance of evidence to prove that 

 many organs of a plant, if subjected to irritation, can respond 

 to it, and not only increase in size by hypertrophy, but 

 materially alter their anatomical structure and develop new 

 processes. Secondly, that these altered states, if the irrita- 

 tion be persisted in, may become hereditary. 



* See Fig. 42, a, (p. 137), which represents the inferior side of an 

 aerial root of PhalcBnopsis amahilis in contact with a surface ; h is that 

 of a root which has penetrated the soil (Organisation dorsiventrale dans 

 les Bacines des Orchidees, par M. E. Janczewski. Ann. des Sci. Nat, 

 ser. 7, torn, ii., p. 55. 



t Malesia, ii. (1884). See Arch. Ital. de Biol., vi. (1885), p. 305. 



X Ann. Jard. Bot. Buit, iii., p. 129 (1882). 

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