646 LECTURE XXXVII. 



the whole plant, so that the motile organs of all the primary petioles curve 

 downwards, those of the secondary petioles forwards, and those of the leaflets 

 forwards and upwards, as shown in Fig. 373 ^. This condition resembles 

 outwardly that of a leaf in the nocturnal position, or the one induced by sudden 

 darkness ; internally, however, it differs from it, because vibration still induces 

 stimulation in leaves which are already in the nocturnal position, as is particularly 

 evident by the relaxing of the lower large motile organ. Mention may here be 

 made of the very important fact, that the nocturnal position induced by dark- 

 ness is connected with an increase of turgescence and rigidity of the organ, 

 whereas it suffers a very marked relaxation under the influence of vibration, so 

 indeed, that a INIimosa leaf stimulated by contact or vibration, may swing loosely to 

 and fro until it again assumes the irritable condition. In the day-time this follows 

 after a few minutes, the leaves assuming the position represented in Fig. 373 ^, 

 whereupon they are again sensitive to vibration. 



In the case of the motile organs of the primary petiole and secondary 

 petioles, in very irritable IMimosae, a slight touch of the hairs on the lower side 

 suffices to induce the movement ; and the lightest touch of the glabrous upper 

 side causes the movement of those of the leaflets. 



The sensitiveness of the Mimosaä depends to a large extent on the elevation 

 of the temperature, and upon the moisture of the air : as these increase, the water 

 contained in the whole plant increases, and particularly the turgescence of the 

 motile organs. With the air at 25-30° C, and sufficient moisture, the irritability 

 of the Mimosa is so great that the mere shaking as one passes by the plant acts as 

 a shock sufficient to irritate it, and its leaves fall and close up ; it is thus almost 

 impossible to lift up and replace a plant in a flower-pot, even with great care, 

 without inducing the effects of stimulation. 



Of pecuHar theoretical interest also is the unusually clear propagation 

 of the stimulus in the Mimosae, which is effective over distances 50 or more 

 centimeters in length. If, for example, one of the terminal leaflets is snipped 

 off, or its motile organ touched, or the sun's rays concentrated to a focus 

 on one of these leaflets, a movement is at once produced, and the opposite 

 leaflet rises almost simultaneously, the neighbouring leaflets then following in 

 pairs, and so on with leaflets further and further away, down to the base 

 of the secondary petiole : after a short pause the lowermost leaflets of a neigh- 

 bouring secondary petiole then begin to fold together, and this proceeds from 

 below upwards, and is repeated by the leaflets of the other secondary petioles, 

 and finally (often only after a long time) the primary petiole of the leaf also falls 

 downwards. If the plant is only irritable to a moderate extent this is all that 

 happens, and the stimulated leaf resumes its normal position after a few minutes ; 

 but in very irritable and quite healthy plants, the irritable movement of the first leaf 

 is followed after a few seconds by the sudden falling down of one of the nearest 

 leaves situated higher or lower on the shoot, and thence onwards, proceeding 

 in series, the irritable movement of all the leaves of the same shoot following as 

 if the plant had been shaken. Thus, in the course of a few minutes all the leaves of 

 a shoot of a vigorous Mimosa may be set in movement, although only a single 

 leaflet had been stimulated originally ; occasionally it happens that individual 



