ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 545 



Perception of the Force of Gravity by Plants.*— F. Darwin 

 gives a valuable discussion on the statolith theory of geotropism, which 

 suggests that the stimulus to geotropic curvatures is supplied by the 

 change in position of the movable starch-grains resulting from an 

 alteration in the position of a plant or plant-organ. Two general con- 

 siderations are of interest. First, analogy with the graviperceptive 

 organs of animals, Kreidl having shown that in the crustacean Palasmon 

 the sense of verticality depends on the pressure of heavy bodies on the 

 inside of certain cavities, or statocysts, formerly believed to be organs of 

 hearing. When the normal particles are replaced by fragments of iron, 

 the animal reacts towards the attraction of a magnet precisely as it 

 former^ reacted towards gravity. Secondly, the specialisation and 

 distribution of the falling bodies in plants. The difference between 

 movable starch-grains (statoplasts) and the ordinary immovable amylo- 

 plasts is striking, and it is hardly possible to doubt that their function 

 is different. In a seedling, Phalaris canariensis, the apical part has 

 only falling starch-grains, while lower down both forms occur, and 

 correspondingly, we find that the seedling is gravi-sensitive throughout, 

 but especially so at the apex. The most striking general fact about the 

 distribution of the statoplasts is their presence in the endodermis. If the 

 endodermis is essentially a tissue of gravi-sensitive cells, we can under- 

 stand why it contains loose starch only as long as the stem is capable of 

 growth curvature. Other cases which strongly suggest a relation 

 between sensitiveness to gravity and presence of statoplasts are the 

 onion, where these occur in the root-cap, the endoderm and punctum of 

 the seedling, but not elsewhere, and the pulvinus of grasses in which 

 statoplasts occur, but not in the haulm. Viscum, on the other hand, is 

 not geotropic, and has no statoplasts. The author has previously 

 shown that the cotyledon of Setaria and Sorghum is the seat of gravita- 

 tion, and it is there that the statoplasts are found. Physiologists have 

 gradually come to accept Charles Darwin's view that the organ of gravi- 

 perception is in the tip of the root, and it is there — generally in the 

 root-cap and there only — that the statoplasts are found. In a large 

 number of plants, such as Algse and Fungi, no statoliths are known to 

 exist ; here we must either accept Noll's view of minute and hitherto 

 unseen statoliths, or suppose a different mechanism such as hydrostatic 

 pressure. 



Reviewing, in some detail, the experimental evidence, the author 

 expresses his opinion that it is not fatal to the statolith theory ; in some 

 cases a revision is necessary, and other considerations must be taken 

 into account. He is impressed with the general, though not the 

 universal, applicability of the statolith theory, and considers that in the 

 case of the higher plants, sensitiveness to the pressure of heavy bodies 

 will be found to be by far the most important, if not the exclusive 

 means, by which gravity is perceived. The stimulus must depend on 

 weight, and since neither the theory of radial pressure nor Noll's 

 supposition of stimulation by small unknown bodies lends itself to 

 experimental inquiry, we are driven, as practical people, to test the 



* Brit. Assoc, 1901, Botanical Section. Address by the President, Francis 

 Darwin, F.R.S. 



