CIRCUMNUTATIOK 



111 



when they are going to sleep at night and un- 

 folding in the morning. The nyctitropic move- 

 ments of leaves and cotyledons are exceedingly 

 complex and are wonderfully diversified. The 

 position which they all assume at night is one 

 by which they are well protected from the 

 effects of radiation into the open sky. Enforced 

 exposure, produced for the sake of experiment, 

 has been proved to be injurious to nyctitropic 

 plants. The folding of the parts together is a 

 further protection against the loss of heat. 

 The course pursued differs only from ordinary 

 circumnutation in its greater extent, and in the 

 acceleration of the movement in the evening 

 and in the_morning. That the nyctitropic move- 

 ment is actuated by heliotropism, is proved by 

 the disturbance of its periodicity, caused by 

 darkening the plant in the daytime, or exposing 

 it to artificial light at night, which is the case 

 also with the ordinary circumnutation of leaves. 

 Unless nyctitropism is admitted to be a modi- 

 fication of circumnutation, the leaves and coty- 

 ledons of nyctitropic plants which describe a 

 single ellipse in the twenty-foar hours do not 

 eircura nutate at all. In other cases the leaves 

 and cotyledons describe several ellipses during 

 the day, the path of one of them in the evening 

 and one in the morning being greatly extended. 

 In plants possessing pulvini the nyctitropic rev- 

 olution is sometimes complicated, accompanied 

 with a twisting movement by which the surfaces 

 of tli3 parts are brought into closer contact, af- 

 fording greater protection. Nyctitropism con- 

 tinues after the parts have attained their full 

 growth only in plants which develop pulmni. 

 The pulvinus is an aggregation of small cells, 

 destitute of chlorophyl, at the base of a stem. 

 The cells of the pulvinus distend and contract 

 after growth has ceased. By reason of the 

 multitude of the cells the movement produced 

 by the extensibility of their walls is more ample. 

 The development of the pulvinus, which con- 

 sists merely of cells whose growth has been 

 arrested at an early age, could be occasioned 

 by very slight causes; and the tendency to 

 develop such a formation, arising frequently 

 as it must from accidental causes, would grow 

 rapidly, in obedience to the law of natural se- 

 lection, under conditions which render such an 

 organ beneficial. The sleep of cotyledons, al- 

 though a very common phenomenon, has never 

 before attracted the notice of botanists so as 

 to elicit more than a passing remark. The ex- 

 tent of the diurnal movement of cotyledons 

 not provided with pulmni is sometimes, though 

 not as a rule, as great as that of pulvinated 

 cotyledons ; but in the case of the former the 

 sleeping habit rarely lasts over a week, while 

 with, the latter it continues for a month or 

 more. The utility of the pulvinus is therefore 

 evident. Pfeffer has examined with the mi- 

 croscope the pulmni of leaves without detect- 

 ing any difference in the structure of the cells 

 of the upper and lower sides, sufficient to ac- 

 count for the upward movement in the leaves 

 of some plants, and the downward movement 



in those of others. Darwin inspected the pul- 

 vini of cotyledons which become erect and 

 those which become pendent at night, but dis- 

 covered no difference in the structure of the 

 opposite halves of either class, although the 

 pulmni of cotyledons are better adapted for 

 observation than those of leaves. The usual 

 explanation of heliotropic phenomena that 

 light checks vegetable growth, a partial illumi- 

 nation occasioning the side favored by dark- 

 ness to grow more rapidly than the lighted 

 side, causing a plant to turn toward the light 

 will not account for many of the varieties of 

 heliotropic and nyctitropic movement ; as many 

 plants which are known to grow best in dark- 

 ness exhibit movements away from the light. 



Heliotropism proper differs from the influ- 

 ence of light upon nyctitropic movements, in 

 that the latter are affected only by the inten- 

 sity of the light, while heliotropic movement 

 depends upon its direction. All heliotropic ef- 

 fects positive heliotropism ; apheliotropism ; 

 diaheliotropism, which causes the parts of plants 

 affected to place themselves transversely to the 

 direction of the source of illumination ; and 

 paraheliotropism, usually called diurnal sleep, 

 which consists in the rising, sinking, or twist- 

 ing of organs when exposed to an intense light 

 in order to protect themselves from its injuri- 

 ous effects are modifications of circumnuta- 

 tion, consisting in an amplification of the ordi- 

 nary movemsnt on one side. Heliotropism, or 

 the property of bending toward a lateral light, 

 is almost universal in the chlorophyl-containing 

 organs of plants; yet when this action proves 

 injurious it can be eliminated or even turned 

 into apheliotropism. Thus the tendrils of many 

 climbing plants are not susceptible to heliotro- 

 pic attraction, and the stems of some, which 

 clirnb by rootlets, such as the ivy, are de- 

 cidedly apheliotropic, which aids them in ob- 

 taining support. Insectivorous plants, which 

 do not live so much by decomposing carbonic 

 acid, and which derive more benefit from an 

 advantageous position of their leaves for cap- 

 turing insects, do not seem to be affected by 

 heliotropism. Heliotropic movements when 

 strongly excited are not attended by lateral mo j 

 tion; but under a gentler stimulus plants circum- 

 nutate visibly when bending toward the light. 

 When light is suddenly admitted to a part 

 which is circumnutating at the moment in the 

 opposite direction, it does not respond to the 

 attraction until it has turned the curve, and 

 then sweeps rapidly toward the light in its 

 return path. There are some movements ex- 

 hibited by isolated genera which have no rela- 

 tion to circumnutation. Such is the move- 

 ment of a mimosa-leaf, when touched, in which 

 a different state of turgescence is produced 

 from that which produces nyotitropism, al- 

 though the action of the leaf is apparently the 

 same. The movement excited in the tentacles 

 of the drosera by the contact of an albuminoid 

 substance is not a mode of circumnutation, nor 

 is the movement of the stamens of the mahonia- 



