CIRCUMNUTATION. 



107 



waxing part of every plant is informed by a 

 principle which causes it to rotate as it grows. 

 These spiral revolutions, usually exceedingly 

 minute, are identical in their nature with the 

 revolving movement of climbing plants, which 

 is called by Sachs revolving nutation. To this 

 principle Darwin gives the name of circumnu- 

 tation. All the other movements of plants, 

 excepting certain peculiar motions of the insec- 

 tivorous species and some other unusual habits, 

 arc modifications of this universal revolving 

 movement. This can be understood when the 

 mechanical cause of the phenomenon is ex- 

 plained. ItT8~the increased turgescence of the 

 cells on one side of the moving part which 

 causes it to bend in the opposite direction. 

 This turgescence is accompanied by an in- 

 creased extensibility of the cell- walls, which 

 in the case of unicellular plants is the element 

 which produces the phenomenon. Circumnu- 

 tation was until recently supposed to be due to 

 the increased growth of the turgescent side. 

 In growing parts such growth follows upon tur- 

 gescence. In such parts as are provided with 

 puloini, otherwise called joints or cushions, 

 turgescence continues to take place in the pul- 

 vinus, and the motion of the part do3s not 

 cease with the cessation of growth. The tur- 

 gescence proceeds from side to side, affecting 

 each portion of the circumference, which be- 

 comes temporarily more convex. Darwin's 

 experiments convinced him that the stems, 

 roots, leaves, flowers, the cotyledons of germi- 

 nating plants, and all the young and growing 

 parts of all plants, are continually circumnu- 

 tating. This revolving movement of the ex- 

 tremities being universally present, the devel- 

 opment of special movements, which consist in 

 slowly tipping more in one direction than in 

 any other during the periodical revolutions, 

 can be explained upon the Darwinian theory 

 of natural selection. The influence of any at- 

 tracting or repellent stimulus becomes more 

 and more active when it proves conducive to 

 the life of the plant. These secondary move- 

 ments are classed by Darwin under the follow- 

 ing names: geotropism, bending toward the 

 center of the earth ; apogeo.tr op ism, bending in 

 the opposite direction from the center of the 

 earth ; diageotropitm, bending in directions 

 more or less transverse to the radius of the 

 earth; heliotropism, bending toward the light; 

 apheliotropism (commonly called negative he- 

 liotropism), bending away from the light ; dia- 

 heliotropism, bending to or away from the light 

 to a position diagonal to the direction of its 

 source ; epinasty, the tendency of the upper 

 surface of a part to grow more rapidly than the 

 lower one, causing the organ to bend down- 

 ward; hyponasty, the tendency of tne lower 

 surface to grow the most rapidly, causing the 

 or<ran to bend upward. 



The Messrs. Darwin subjected the growing 

 roots of many different species to close obser- 

 vation, and discovered through delicate experi- 

 ments the manner in which the radicle of a plant 



finds a passage downward through the soil. 

 In a germinating seed the radicle is the first 

 part which emerges. It immediately bends 

 downward in the direction of gravitation. 

 The tip of the radicle is constantly circumnu- 

 tating, and consequently finds its way into the 

 crevices of the soil, or down into holes left by 

 decayed roots or made by earthworms or larvae. 

 The tip is the portion which is sensitive to 

 gravitation or geotropism, and as was verified 

 by numerous experiments is repelled by con- 

 tact with any hard substance, and attracted by 

 moisture. When the seed is covered with soil 

 which is compact enough to offer sufficient re- 

 sistance, the radicle, following the path found 

 for it by the sensitive and continually gyrating 

 apex, forces its way like a wedge in virtue of 

 its longitudinal and lateral growth. When the 

 seed germinates on the surface of the ground 

 or is forced up by the descending root, the 

 slender root-hairs which are sent out from the 

 upper part of the radicle usually encounter 

 stones or other solid objects to which they firm- 

 ly affix themselves, and thus afford a resistance 

 which enables the embryonic root to pene- 

 trate the eartii. The outer surface of the cel- 

 lulose walls of the filaments liquefies upon con- 

 tact with a hard body and then sets hard in 

 the air. The object of this process is probably 

 not to assist the radicle to penetrate the earth, 

 but to bring the hairs into close contact with 

 the grains of the soil, so that they can absorb 

 the layer of water and dissolved matter sur- 

 rounding them. The apex of the radicle can 

 not assist in the penetration, but only guide 

 the way. Acting under the influence of geo- 

 tropism, strong-growing roots, like that of the 

 bean, do not exert a force of more than one 

 gramme, as Sachs proved by attaching weights 

 suspended over a pulley to radicles held in a 

 horizontal position. The lower extremity of 

 the growing part of the radicle responds to the 

 influence of the apex. The longitudinal press- 

 ure of the terminal growing part, which is 

 very rigid, Darwin fcund, in the case of the 

 bean, to be equal to a quarter of a pound when 

 free ; but it is doubtless much greater when 

 held in place by packed earth on every side. 

 The transverse growth of the root exerts a force 

 equal to several pounds. The sensitiveness of 

 the apex of the radicle to hard bodies was 

 proved by affixing with shellac minute squares 

 of cardboard to the tips of the radicles of seed- 

 lings growing free in glass jars. The apex, in 

 the cases of the bean, the pea, the oak, maize, 

 and all the other species experimented upon, 

 would bend to the side opposite to the attached 

 object for one or two days, the radicle some- 

 times forming a complete loop or even two. 

 In the end the repellent influence would cease 

 to act, and the radicle would straighten out 

 again in obedience to geotropic attraction. 

 Very minute bodies, such as a bead of shel- 

 lac weighing '33 milligramme, sometimes caused 

 this curious curvature. When the roots were 

 placed in a horizontal position, and pieces 



