IN THE MOTOR ORGANS OF LEAVES. 



10* 



ought to occur in connection with exposure to any conditions implying considerable 

 disturbance of pre-existing relations between supply and loss, and these ought to be 

 rapidly carried out where the conditions are such as to imply rapid dist urban- e. It is 

 easy to acquire evidence that this theoretical requiiement is amply met by actual 



facts. The phenomena which have just been described as arising in connection with 

 the transfer of plants from saturated to relatively dry air are, of course, an example of 

 movements originating under such conditions, and one has not to look far for others of 

 a similar nature. It has already been pointed out that when plants which have been 

 for some time in the shade, and whoso pinnules have attained a maximal degree o 

 expansion are exposed to strong, direct sunshine, either as the result of tho natural 

 diurnal alterations in distribution of the latter, or of transfer from one locality tc 

 another, gradual movements of e'evation and convergence set IB in the pinnules, so that 

 the leaves presently become much less fully expanded than they provionsl weir. If 

 atmospheric humidity be very low and the soil very dry, tlie movement- are continued 

 until the pinnules have assumed their maximal nocturnal position. If, on tho other hand, 

 atmospheric and telluric aridity be not very great, the displacement is only a partial 

 one, but it is always sufficient to cause the appearance of iosolatcd plants to differ 

 strikingly from shaded ones in their immediate vicinity. "Whore atmospheric and telluii 

 humidity are alike moderate, the activity of root-supply is incapable of mumtsinii 

 active turgescence in the tertiary pulvini at the same level as when transpiratory 

 loss was considerably less, but it is capable of maintaining enough turgsscenee to 

 prevent the pinnules from passing on into the fully developed nocturnal position. 

 But when atmospheric humidity falls very low, and when tho supply of water in 

 the soil is very small, the disproportion between root-supply and transpiratory loss 

 becomes excessive, and the movements are carried out in maximal degree. The 

 phenomena are piecisely parallel to those occurring in the case of other nyctitropic 

 leaves under similar circumstances, but the movements are more readily induced and 

 more rapidly conducted than they are in these because of the exceptional facilities for 

 transpiratory loss and for rapid redistribution of liquid which the tissues of the 

 leaves present. Parallel phenomena of exceptionally rapid manifestation of the effects of 



eased transpiratory loss manifest them 



th ceitain non-nyetitropic 



leaves. An exposure to strong, direct sunshine, as brief or almost as brief as that 

 sufficing to give rise to sensible elevation of the pinnules of Mimosa pudica will induce 

 perceptible general wilting in the leaves of certain aroids, such as " Pothos violacea" of 

 gardeners, unless both soil and air are loaded with moisture. In the case of u Pothos 

 violacea" we have evidence of general and evenly diffused loss in turgescence; in that 

 of Mimosa pudica of specially localised loss in turgescence as the result of exposure to 

 increased transpiratory loss; and in both cases cases alike the loss in turgescence is excep- 

 tionally rapid. No one would dream of ascribing the wilting of the leaves of "Pathos 

 violacea" to active contraction of the protoplasts of the tissues; but the firmly fixed 

 belief in the existence of special functional endowments in the protoplasts of the motor 



of Mimosa pudica might lead to a belief that the parallel phenomena of the 



to some directly stimulant action of the suns 



it" not that precisely similar ones manifest themselves where increased 



inspiratory loss arises quite independently of any alterations in conditions of illumi- 



nation, and that the results following exposure to direct sunshine vary » degiee with 



the extent to which facilities for transpiratory loss are present. 



organs 



of the pinnules was owing 



ays, were 



