IN THE MOTOR ORGANS OF LEAVES. 



11 



h 



in the mature axis, epidermis has been replaced by strata of corky time 



protecting the deeper portions of hypoderma with a dense impermeable OOtt, and 

 there is no system of great intercellular channels, like that in the hypoderma of the 

 petioles to afford accommodation to fluids escaping from the interior of the tissue 

 elements (Plate V, Figs. 8, 15). The phenomena are the parallels of those cecum,* in 

 cases of exposure of shoots to the action of chloroform-vapour in hieh suptrm-ial 

 exudation of liquid occurs in young, green shoots and is entirely aWnt in mature 



woody ones. Running parallel with these differences in local effects, w, find dill _ 

 in respect to the phenomena of movement following the treatment. In the case of the 

 young, soft shoots movements may occur both centripetally and eentrifagally along the 

 course of the axis, just as they occur both centrifugally and cent ri petal ly after tl , 

 "ppHcation of heat to points in the course of petioles; but in old, woody shoots tin 7 



occur in centrifugal direction alone, unless the point at which boat is npplii d l>c im- 

 mediately in the neighbourhood of the first leaf in the centripetal direction, in which 

 case movements may occur in it, but in it alone in that direction (vide appendix F> 



Now, it is not easy to account for these phenomena of distribution of movement in 

 mature shoots, save as the result of local obstruction to water-supply. Where the shoots 

 are still young and soft, heating leads, not merely to sudden local expansion of the 

 gaseous contents of the tissues, but to a sudden loss of water of turgescence from the 

 outer surface, precisely similar to that attending immersion in boiling water or death die 

 to exposure to the vapour of chloroform, and in the case of petioles, although any 

 free escape of liquid of turgescence does not occur on the outer surface, ample accom- 

 modation for such liquid is afforded by the great hypoderma], intercellular channels; 

 but in the hard dense tissues of old axes the suberous strata prevent superficial 

 exudation, and the density of the texture throughout the hypoderma affords no 

 space for any appreciable bulk of liquid save within the interior of the cells 

 and hence any sudden rise in temperature is practically expended in giving rise to 

 corresponding expansion of the gaseous contents of • the water-conducting system. 

 Where any appreciable exudative discharge of liquid of turgescence takes 

 in the case of young soft shoots, two factors affecting water-supply come int.. 

 play; there is both sudden expansion of the gaseous contents of the water-con- 

 ducting system, and a sudden establishment of an abnormal local drain of liquid. 

 Any obstructive action of the former can only toll in interfering with water-supply 

 in a centrifugal direction unless in the immediate neighbourhood of the site in which 

 it originates, but the abnormal local drain attending exudation must, of course he 

 capable of telling both centrifugally and centripetal ly. There are thus reasons why 

 movements depending on sudden depressive alterations in water-supply should in such cases 

 make their appearance in both directions. But where, as in old shoots, no local exudation 

 occurs, where no local drain is established, we have to deal with obstruction alone 

 which can only tell centrifugally, or, in extremely localised fashion, centripetally, and 

 consequently, it is only centrifugally, or, in the immediate neighbourhood of the site of 

 origination, centripetally, that any movements connected with sudden diminution of water- 

 supply will occur. Where local obstruction and drain coincide, movements related to de- 

 fective supply of water may occur in both directions; where obstruction alone is present 

 they can only occur centrifugally or in the immediate neighbourhood of the site in whicl 

 the obstruction originates, because everywhere beyond this area in the centripetal direetioi 



as 



1 



Ann. Roy. Bot. Gard. Calcutta Yol. VI. 



