IN THE MOTOE ORGANS OF LEAVES. 



»: 



tissues on section, is entirely derived from the system of >eive-tubes which lie recrarda 



e ~ ... a* 



constituting a specific, mechanical nervous system for the propagation of stimulant 

 impulses inducing active contraction of the protoplasts of the meter organ- but, even if i 

 be so, this does not affect the question of the instability of equilibrium between gen ft] 

 supply and loss of water. The maintenance of high liquid u usion within the en 

 tubes demands a constant supply of water to satisfy the excessive osmotic capacit 

 of the cell-sap which they contain. Their excels ive tnrgeeoenco only impliet 

 an additional drain upon the general stock of water which is also subject to 

 excessive transpiratory loss. The presence of any extensive system of ii uo elements 

 of exceptionally high osmotic capacities, such as the seivo tubes in the bast of Mimm 

 pudica, or the laticiferous tubes of Enphrbia antt qmnm (Rale V1J, Fi$ »), can only 

 serve to render harder the struggle which other le highly endowed elements ha\< 

 to maintain with transpiratory loss in tho effort to retain their tnrgi'seence. The 

 stream of water along the course of the water-conducting tissues of Mimota jiiui a 

 must in any case be an extremely active one if turgescenco is to be fully maintained 

 under conditions favouring active transpiratory loss, no matter whether the drain upon 

 it be carried on by all the active tissues in like amount or by a certain class of them 

 in excessive amount. In either case the facilities for transpiratory loss renin in exn -ive. 

 and the demand for water to satisfy the osmotic capacities of the tissu. 9 is very hig 

 so that turgescence can only be fully maintained under external conditions favouring the 

 occurrence of transpiration by means of a very active current along the >urse <d 

 the water-conducting system. Whenever atmospheric conditions are such as to favour 

 transpiration, turgescence can, therefore, be fully maintained at a high level onh 

 by means of proportionately active root-supply of water; and any sudden inn* is- 

 in transpiratory loss or diminution in root-supply must nccdBurily disturb the 

 pre-existing equilibrium and give rise to a loss in turgescence which will, oi 

 course, manifest itself most rapidly and readily in those tissues which present tin 

 greatest textural facilities for the escape of water. Loss in tuig. scence originating 

 in any disturbance between the relations of general supply and loss of water will 

 not occur with equal rapidity everywhere; for the rate of loss must be affected on the 

 one hand by the osmotic capacities of the various tissue elements, and on tho other 

 by the structural facilities which they provide for the escape of liquid; and where no 

 great difference in osmotic properties is present, those tissues which present the greater 

 structural facilities for the escape of liquid from the interior of their turgescent elements 

 will lose turgescence most rapidly, and with this will become relatively weaker than 

 they previously were. But in the motor organs of Mimosa pudica, and especially in 

 those of them which are most subject to rapid movements, we have to deal with 

 opposing masses of tissue whictf' differ very greatly from one another in the textural 

 facilities which they provide for rapid filtrative escape of liquid. Any sudden increase 

 in transpiratory loss or decrease in the activity of root-supply must necessarily give 

 rise to decreased liquid pressure throughout the water-conducting system generally, 

 and this in its turn to an increased tendency to filtration from tho interior of turgid 

 elements. But the rate of filtration in particular elements must be affected by the 

 structural features which they present, and with this corresponding alterations in their 

 relative strength must be established. Where filtrative facilities are present in very 

 different degrees in opposing masses of tissue, as is the case in the primary and tertiary 



Ann. Roy. Bot. Gard. Calcutta Vol. VI. 



