LECTURE XXXVn. 



THE IRRITABILITY OF MIMOSA AND OTHER PLANTS. 



Like several other Mimosai and Oxalidae, the Sensitive-plant {Mimosa ptidica) 

 is noted for the fact that the motile organs of its leaves are irritable not only in 

 response to the various stimuli mentioned in the previous lecture, but also to small 

 vibrations and other disturbances. These are the most easily perceived and therefore 

 the longest known phenomena of irritability, and indeed up to the present century 

 they were regarded as the only ones in the vegetable kingdom. The effects of 

 stimulation and the mode in which those effects are produced in this case have now 

 been studied so often, so thoroughly, and with such good results, that we may regard 

 them as affording so far the most solid foundation in the whole province of the 

 phenomena of irritability, so that not only a series of organs which behave similarly, 

 but in fact almost all other phenomena of irritability, become more or less in- 

 telligible by means of them. 



Mimosa pudica, a Leguminous plant belonging to the widely spread tropical 

 family Mimoseae, is a native of Brazil, and is now commonly met with in the East 

 Indies and other tropical countries ; even with us it produces abundance of seeds 

 capable of germination, and this not only in pots and cultivated in the window, but 

 even in the open, so that anyone may grow this remarkable plant, and with little 

 trouble observe the phenomena of irritability now to be described. Growing in 

 the open, especially in bright sunshine, it forms several vigorous foliage-shoots, 

 often 60-80 cm. long, which lie prostrate on the soil; in a chamber, on the 

 contrary, i.e. in a feebler light, the principal shoot grows erect, and only a* few 

 lateral shoots come out obliquely from below. On each shoot-axis there are 

 6-10 doubly compound foliage-leaves; on a petiole 4-8 cm. long, we find two, 

 or a couple of pairs of secondary petioles each 4-5 cm. in length, and each of 

 which bears 15-25 pairs of small leaflets. Each leaflet is about 5-10 mm. long 

 and 1-5-2 mm. broad (see Fig. 373 A). All these parts are mutually connected by 

 well developed motile organs. Each leaflet is situated directly upon a motile 

 organ, o-4-o-6 mm. in length, joined to the secondary petiole; the four secondary 

 petioles in their turn are connected to the end of the primary leaf-stalk by a some- 

 what larger organ which is 2-3 mm. long and about i mm. thick. The base of the 

 primary leaf-stalk itself is transformed into a motile organ 4-5 mm. long and 2-2-5 

 mm. thick. 



The structure of these motile organs in general agrees with that of those of the 

 Bean and Wood Sorrel which have been described in the preceding lecture : each 



