EFFECT OF EXTEENAL AGENCIES ON GEOWTH. 609 



organs to descend, others to ascend, according as growth is favoured or 

 retarded on the side next to the earth. When growth is retarded on the 

 side next the earth, a curvature, the concavity of which is directed to the 

 earth, ensues, and the extreme growing-point is thus directed downwards, 

 as in the case of the gills of Fungi, and most roots, ^~\ Such organs 

 are spoken of as positively yeutropic, the condition being strictly analogous 

 to positive heliotropisrn. On the other hand, when growth is accelerated 

 on the side next the earth, the organ becomes so curved as to present its 

 convexity to the earth, and its terminal growing-point is directed up- 

 wards, as in the following diagram, where the horizontal line represents 

 the earth, ^-J_. Such organs as stems &c. are negatively c/eotropic, and are 

 analogous to negatively heliotropic organs. The direction assumed by 



g 'owing organs depends on the relation between gravitation and tension, 

 the tension resulting from turgescence is equal on ah 1 sides, the root 

 will always grow in a certain direction even against gravity ; but if the 

 internal tension of the tissues be greater in one direction than in another, 

 a curvature will result independently of the action of gravity, the under- 

 side of the curved part will grow more freely than the upper end ; in 

 consequence, the growing- point will be directed upwards. Kraus remarks, 

 with reference to this subject, that geotropic curvatures, positive or nega- 

 tive, are regulated by the same cause, the more abundant flow of nutri- 

 tive juice to the lower part of any organ being determined by gravitation, 

 while the direction of the curvature is determined by the degree of 

 turgescence and tension of the tissues. The subject of the inverse direc- 

 tion of roots and stems stands in need, however, of much further eluci- 

 dation. 



Effects of Tension. These have been already incidentally alluded to ; 

 but their influence on the growth of plants may be better understood if 

 it be remembered that the cells are elastic bags subject to be stretched 

 and compressed, with consequent alterations of shape, varying according 

 to circumstances and according as the pressure is from without or from 

 within. Pressure from within, turgescence, is regulated by hydrostatic 

 laws ; that from without is caused by many varied circumstances con- 

 nected with growth and the movements of fluid. As the absorption of 

 water is known to be more or less periodic, so also the tension of cells, 

 and the movements consequent on it, are periodic. Organs at first 

 homogeneous as to structure absorb an equal amount of water through- 

 out ; but as the structure alters in character at different places, the rate 

 and degree of absorption vary, and the consequent tension also varies in 

 different parts of the plant. Sachs shows that there is an intimate de- 

 pendence and correlation of growth and tension. Daily periodicity of 

 growth coincides with the daily periodicity of tension ; if the former be 

 dependent on changes of temperature and light, the latter is probably 

 so also. One of the simplest of these cases is the separation of the divided 

 portions of a stem when cut lengthwise. Take almost any young grow- 

 ing shoot, preferably one hollow in the interior (the flower-stalk of a 

 Dandelion, for instance), and cut it down lengthwise for a short distance; 

 immediately the separated portions diverge one from the other. The 

 appearance thus presented is like that of the letter V ; or when, as often 

 happens, the. edges roll up, we have an appearance resembling that of the 

 zodiacal sign of Aries, Y. Again, let the layers of bark and wood-tissue 



