13 



4.M ON FLUCTUATIONS IN TURGESCENCE. 



The 



evening depression of atmospheric humidity 



)t 



and the consequent increased facility for transpi- 

 ratory loss must, of course, tend to facilitate the occurrence and diffusion of rapid movements 

 connected with fluctuations in turgescence, but at the same time it tends to give rise to a very consider- 

 able reduction in distal leverage, which must more or less equilibrate the increased instability connected 

 with increased transpiratory loss, so that on the whole the evening rise in petiolar "irritability" may 

 be fairly credited to decreased retentive power in the pulvinar tissues incident on diminished osmotio 

 property in the cell-sap. That this is, at all events, the main determinant of the rise in "irritability 

 which normally presents itself in the evening is, moreover, demonstrated by the phenomena which present 

 themselves during periods of prolonged and more or less continuous rainfall, when both soil and air are 

 practically saturated and transpiratory loss is necessarily reduced to a minimum. Under such circumstances 

 it will be found that, whilst amputation of the tips of terminal pinnules during the day, and whilst the 

 leaves are in maximal diurnal position, is followed only by very limited aud imperfect movements of 

 piuimlar elevation, purely confined to the injured pinna and unaccompanied by any movements of petiolar 

 depression save in the case of leaves which are heavily loaded with adherent water, a similar injury in 

 the evening and when the leaves have already completely assumed the nocturnal position is constantly 

 followed by the deepest petiolar depression quite apart from the presence of any extrinsic distal loading. 

 The effects of simple mechanical "stimulation" also vary very conspicuously in degree at different times 

 of day. The Laves of seedling plants in the morning and early forenoon, so long as they are not 

 exposed to the influence of direct sunshine and its attendant excess in transpiratory loss, are very highly 

 elevated and exceedingly irresponsive to mechanical impulses, whilst in the evening, when the pinnules 

 and pinnee are in their maximal nocturnal position, the primary petioles remain very highly elevated, 

 but very readily undergo the deepest depression on contact. 



In the previous experiments we have been dealing with cases in which we have evidence of increase 

 in "irritability" coincident with decreased assimilatory activity connected with the removal of solar 

 stimulation; but parallel phenomena present themselves in cases in which protoplasmic activity is depressed, 

 not by the absence of normal stimuli, but in consequence of exposure to the influence of anaesthetics. 

 In order to the proper comprehension and interpretation of such phenomena it is necessary to have some 

 acquaintance with the effects which follow exposure to simple desiccated atmospheres and simple exposure 

 to anaesthetic vapours, as the evidences of increased "irritability" connected with the latter manifest 

 themselves most clearly in cases of sudden exposure to a desiccated atmosphere containing vapour 

 of chloroform. The data which follow are accordingly derived from three distinct sets of 

 experiments. In the first of these plants were exposed to the atmosphere of a hermetically sealed 

 chamber containing a vessel of pure sulphuric acid; in the second they were exposed to a humid 

 atmosphere containing chloroform vapour ; in the third they were exposed to the desiccated atmosphere 

 of a chamber containing both chloroform and pure sulphuric acid. 



Experiment IV.— A pot-plant, in a pot which had been waxed and luted as if for experiments on 

 transpiratory loss, was enclosed in a hermetically sealed chamber containing a vessel of pure sulphuric 

 acid aud exposed to diffused sunlight. The leaves passed into the maximal diurnal position, but were 

 extremely "irritable," movements attending the slightest mechanical disturbance, and the momentary 

 application of flame to the tips of terminal pinnules being followed, not merely by complete movements 

 throughout the injured leaf, but by propagation of movements to several other leaves. The soil in the 

 p>t was sufficiently moist, and the proportion of root-surfaces to foliar surfaces was sufficient, to permit 

 of the maintenance of the maximal diurnal position in spite of very active transpiratory loss; but the 

 equilibrium was a very unstable one, aud consequently movements were very readily induced under the 



tissues of the motor organs, or any 

 sudden increase in transpiratory loss, or abnormal escape of liquid from the tissues incident on injury. 



Experiment V.— A pot-plant was set within a common hermetically sealed chamber containing a 

 vessel including a plug of cotton-wool soaked in chloroform. The leaves slowly and gradually assumed 

 the nocturnal position more or less completely, and after some time death of the entire plant occurred. 

 In this case the assumption of the nocturnal position was clearly simply Owing to the action of the 

 anesthetic in depressing assimilatory activity ; for the plant was throughout exposed to abundant 

 diffused sunlight, and the atmosphere of the chamber must very soon have become more or less 

 saturated owing to transpiration from the leaves and evaporation from the moist soil in the open pot. 



influence of any factors either favouring increased filtration in the 





