322 BOTANY 



PART I 



the usual day-position and to assume a ditferent one ; this is either externally 

 similar to the night-position or is diametrically opposite to this. Thus the leaflets 

 of Eobinia are bent downwards at night, in dilfused daylight they are spread out 

 flat, while in the hot mid-day sunlight they stand vertical. This so-called diurnal- 

 sleep is only found in leaves with pulvini and is brought about in a dift'erent way 

 to the evening change of position ; there is no increase of turgescence but a condition 

 of flaccidity. 



When a jjlant has carried out regular nyctinastic movements for a long period 

 under the influence of the alternation of day and night, the periodic movements con- 

 tinue for some days in constant liglit or constant darkness. In some plants it is 

 |)0ssible to bring about experimentally a shorter or longer period of change than the 

 usual one of twenty-four hours ; this new periodicity also shows an after effect. 



(b) Chemonasty (i"'*) 



Chemonasty bears the same relation to chemotropism as i)liotonasty 

 does to phototropism. From whatever side a chemical stimulus (such as 

 the vapour of ether, chloroform, or ammonia) acts on a sensitive tendril 

 the same side of the latter always becomes concave ; this is the side 

 which is especially sensitive to thigmotropic stimulation. 



These chemonastic curvatures of tendrils are evidently of no use to the plants. 

 Thus natural selection cannot have played a part in bringing about these properties 

 of tendrils. The same is the case for the nastic movements of tendrils whicli take 

 place on wounding and on rise of temperature (traumatonasty, thermonasty). On 

 the other hand chemonastic movements play an important part in some insectivorous 

 plants. 



Very striking chemonastic movements are exhibited by the tentacles 

 of Drosera (Fig. 198). On chemical stimulation these curve so that 

 their upper sides become concave and the glandular heads are thus 

 broutrht towards the centre of the circular leaf. Such substances as 

 albumen, phosphates, etc., which Drosera can use as food, serve as stimuli 

 (jD. 228) ; so also can indifferent and even poisonous substances. Often 

 minimal traces of these substances [e.g. 0'0004 mgr. of ammonium 

 phosphate) suffice to bring about the irritable movement ; when the 

 stimulus is applied to the summit of the tentacle it leads to the 

 curvature at the base of the latter. There is thus in this case as in 

 certain phototropic curvatures, but even more clearly than in these, 

 a separation between the organ of perception which receives the 

 stimulus and the motile organ that effects the movement. The 

 stinudus received by the head of the tentacle must be conducted to 

 the base of the latter. 



The curvature resulting from growth may be carried out in the same way as in 

 tendrils. Furtlier the tentacles of JJruscra in common with tendrils can exhibit 

 tliigmonastic and tliermonastic reactions. Doubtless, however, their chemonastic 

 irritability is the main and most important one. Clicmical stimuli are concerned in 

 the movements of other insectivorous plants, e.g. Dlonaea (p. 2'28) and Piiujuicula. 



