108 



CIRCUMNUTATION. 



of card fastened to the tips so as to influence 

 them to bend in opposition to geotropism, this 

 latter force could not be overcome; acting 

 then, according to Darwin's explanation, di- 

 rectly, while the initial action of the affixed 

 bodies is only opposed by the influence of 

 geotropism acting obliquely, and consequently 

 much more feebly. When one side of the tip 

 was very slightly cauterized with dry caustic, 

 it behaved in the same way, causing the radicle 

 to curve in the opposite direction. The tips of 

 radicles were also found to be sensitive to moist 

 air, and to bend toward the source of moisture. 

 A small section of the radicle above the apex 

 is sensitive to the contact of hard bodies, but 

 acts in the opposite way, bending toward and 

 around an object like the tendril of a vine. 

 This is a very useful property, enabling the 

 root to curve quickly around an obstacle and 

 regain its vertical course more rapidly than the 

 tip could under the influence of geotropism 

 alone. The sensitive tip of a radicle has a 

 length of from 1 to 1-5 millimetres. The grow- 

 ing part which curves in the direction given it 

 by the tip is the part of most rapid growth, 

 and is from six or seven to twelve millimetres 

 long. The tip and the basal part grow very 

 slowly and bend very little. The roots of most 

 plants are sensitive to light, turning sometimes 

 toward and sometimes away from its source. 

 This does not appear to be of any service, and 

 is probably an indirect result of the tips being 

 highly sensitive to other stimuli. The second- 

 ary radicles of plants are diageotropic in their 

 growth, growing out horizontally or with a 

 slight incline downward, and, if displaced, will 

 resume their original direction. Their tips are 

 sensitive to contact, and have been observed to 

 circumnutate. These emit a third system of 

 roots which are not influenced by geotropism. 

 "When the main radicle is destroyed or com- 

 pressed in such a way that the sap can not 

 freely enter it, two or three of the secondary 

 radicles nearest it become geotropic and shoot 

 down perpendicularly into the earth. An anal- 

 ogous behavior is observable when the main 

 shoot of a tree is killed or injured on the part 

 of the nearest side-branches, which are im- 

 mediately rendered apogeotropic. These phe- 

 nomena, as well as the apogeotropic growth of 

 shoots which spring from branches that have 

 been injured by certain insects, Darwin ex- 

 plains by the principle of reversion, the dis- 

 turbance which brings it into play in the case 

 of the secondary roots or steins being the in- 

 creased flow of sap. 



The fact that the sensitiveness of the radicles 

 of seedlings to gravitation is confined to the 

 apex was proved by an interesting experiment. 

 The roots of beans were placed for an hour or 

 an hour and a half in a horizontal position, so 

 that the force of geotropism could act most pow- 

 erfully upon them. The tips were then ampu- 

 tated, and the radicles were placed in the verti- 

 cal position. The influence communicated to 

 the growing oart soon began to act, and the 



root became bent and grew for two or three 

 days in a horizontal direction, until a new tip 

 formed which brought it again under the in- 

 fluence of geotropism, and again caused it to 

 curve downward. Ciesielski first performed 

 this experiment, and discovered the power of 

 communicating the received influence thus 

 clearly shown to belong to the vegetative 

 point of the radicle. Sachs repeated his ex- 

 periments with roots of the pea, bean, and len- 

 til, but did not obtain corroborative results. 

 Darwin tried it with a greater variety of plants, 

 and found that a large majority of the ampu- 

 tated roots behaved as described. The part of 

 the radicle which is chiefly influenced by the 

 sensitive apex is three to six millimetres above 

 it. The effect of the transmitted tendency re- 

 ceived by the growing part must be observed 

 within forty-eight hours after the amputation, 

 as a regeneration of the root -cap and vegeta- 

 tive point often occurs as soon as that. The 

 fact that it is the tip alone which is acted upon 

 by gravity, and that this has the power of in- 

 fluencing the direction of the growth of the 

 superior part of the radicle, shows that the 

 roots of plants are not made to grow down- 

 ward by the mechanical force of gravitation 

 acting directly upon their tissues. The action 

 of gravity on the root is similar to the effect 

 of pressure upon a lowly-organized animal 

 which causes it to draw away. The tip of 

 the radicle seems to possess more wonderful 

 properties than any other portion of the struc- 

 ture of plants, and to be more delicately sen- 

 sitive, and sensitive to a greater variety of 

 stimuli, than any 'other part. The important 

 function it has to perform in enabling the 

 root to penetrate the soil, makes it necessary 

 that it should be endowed with such remark- 

 able powers. Darwin likens the action of 

 the tip of the radicle in directing the move- 

 ments of the adjoining parts to that of the 

 brain of the lower animals, which, analogously 

 to the apex of the root, is situated in the ante- 

 rior part of the body as it moves. The tip of 

 the cotyledon when affected by light appears 

 to transmit motion to the adjoining parts like- 

 wise, and also exhibits a sensitiveness and va- 

 riety of adaptations almost comparable to the 

 functions of the nervous structure of animals. 



With dicotyledonous plants the hypocotyl 

 protrudes first after the radicle, and makes its 

 way upward to the surface ; or, if the cotyle- 

 dons are hypogean, that is, develop below the 

 surface, it is the epicotyls, or sometimes the 

 petioles of the cotyledons, or even those of the 

 first true leaves in some cases, which emerge 

 from the seed-coats and rise to the surface. 

 Whichever of these organs it is that conducts 

 the sub-aerial portion of the plant to the light, 

 it invariably assumes an arched form, the stem 

 while forcing its way through the seed-coats 

 and then vertically upward through the soil 

 being bowed into an elbow, probably for the 

 purpose of protecting the tender apex from 

 abrasion. As soon as it emerges into the air 



