IN THE MOTOR OBGANS OP LEAVES. 



105 







are essentially depandent on active stimulation an 1 contraction of the protoplasts of 



indeed on the groundless 



exposure to hot and dry air 



acts as a specific stimulant of the protoplasts of the motor organs of the tertiary pui 



and exposure to cool, moist air as a specific stimulant to those of the primary on. s. But 

 if we accept the theory that the movements are dependent on conditions affecting supply 

 and loss of fluid, the sequence admits of ready explanation. Bttdden tranter to » 

 relatively warm and dry atmosphere must give rise to increased tran>niratory J< > am] 





ory joss 

 must establish a drain upon all the tumescent elements of the tissues to which tl 



were not previouly exposed, so that, unless root-supply sustains an equivalent increase, a 

 general loss of tumescence must tend to be established. The actual loss in turgeecen. 

 which will occur in individual masses of tissue will be rapid and extensive iu direct 

 relation to the facilities which they provide for the escape of liquid from the interior 

 of the cell-elements of which they are composed. But the tissue-elemcnis iu tl 

 masses of tissue which make for the diurnal position in the primary and t< t 

 pulvini afford much greater facilities for the escape of liquid, than those which mak< 

 for the nocturnal position. Hence a relative weakening of the former must occur, and 



lOS 



in 



with this, if no interfering factor come in to modify the result, movements towards tli 

 nocturnal position must necessarily ensue. In the case of the tertiary pulvini, nothing 

 comes in to interfere with the execution of such movements, and they are according 

 carried out at rates varying with the rapidity at which the weakening loss in turgescen 

 occurs within the pulvinar tissues making for the diurnal position. But, in the priman 

 pulvini, the local weakening is accompanied by coincident rapid decrease in distal 

 leverage which discounts and more than discounts it. The mass of tissue in the prima n 

 pulvinus which makes for elevation of the petiole is weakened in so far as respects it 

 local pulvinar opponent, but is strengthened in respect to the total resistance which ii 

 has to overcome,' and hence elevation and not depression occurs. But when the plain 

 is once more suddenly transferred to its originally cool and moist environment, a sudd* u 

 check is put on the activity of transpiratory loss, and with this no farther elevation of 



the pinnules occurs, and distal leverage, in place of undergoing any farther decrease 

 begins to increase. Simultaneously another important factor affecting the general dis- 

 tribution of fluids throughout the tissues comes into play. The rapid cooling followin_ 

 transfer to the cool air necessarily implies rapid contraction in all the gaseous content > 

 of the water- conducting tissues, and this must tend to give rise to a back draught * 

 the liquid contained within the actively turgescent ones and, therefore, to enhance 

 filtrative loss from them. Ordinarily the effect of this on the pinnules is done awa} 

 with by the coincident cessation of active transpiration, and, consequently as a rule, no 

 appreciable movement occurs in them, but in the case of the primary pulvinus nothing 



its action. The cessation of transpiratory loss does not here 



in 



interfere with 



tell against the occurrence of movement, but directly in its favour in consequence of the 

 increase in distal leverage which it necessarily involves, and, consequently, depression of 

 the primary petioles naturally tends to occur. As will be presently demonstrated, a 

 sudden expansion of the gaseous contents of the water-conducting tissues may also 

 serve to occasion sudden movements, apparently as the result of the obstruction to 

 onward flow of water which it induces if pushed beyond certain limits; but any 

 obstruction which arises in cases like the above on the introduction of the plant into 

 the heated atmosphere is, unless it be present in very high degree, unable to determine 



Ann. Roy. Bot. Gard. Calcuita Vol. VI 



