296 



HUTCHINSON'S POPULAR BOTANY 



FIG. 361. STINGING 



HAIR OF STINGING 



NETTLE. 



her vegetable proteges against excessive loss of heat. 

 The " sleep " position of leaves is, in fact, a protective 

 arrangement. By folding themselves together and as- 

 suming, as far as possible, a vertical position, radiation 

 is materially checked, and thus the plants undergo no 

 serious fall of temperature daring the night. We must 

 not be misled "by this popular term "sleep" into sup- 

 posing that the nutritive processes of the plant are 

 suspended at this time. " The drooping position as- 

 sumed by the leaflets of Oxctlis is simply protective : there 

 is no correlation between the assumption of the drooping 

 position and the temporary loss of the power of assimila- 

 tion. Preparations made at night from the leaves of 

 Oxalis when in the drooping nyctitropic position show a 

 normally active power of assimilation, and the same is 

 the case with leaflets of Mimosa, The movements per- 

 formed in assuming the nocturnal nyctitropic position 

 of certain ' sleep ' plants are not accompanied by any 

 corresponding internal changes or alterations in the power of assimilation. 

 In this respect the sleep of plants is more external and apparent than, 

 internal arid real ' ; (A. J. Ewart, B.Sc., in Journ. Linn. Soc. [Botany], 

 vol. xxxi., 1896). 



Plants of the great Leguminous order, to which the Acacias, Mimosas, 

 Peas, and Trefoils belong, exhibit the phenomenon of which we are treating 

 in a very striking manner. The Wood-sorrels (Oxalis acetosella, corniculuta, 

 and stricta], also, are extremely sensitive to changes of temperature, folding 

 down their leaves even in the daytime if rain threatens (figs. 353, 354), while 

 a blow from a stick will cause them to shrink together with affecting 

 suddenness. Sensitiveness is carried to an extreme in a tropical species of 

 this genus, Oxalis sensitiva, concerning which it is affirmed that even the 

 disturbances of the air caused by the approach of man are sufficient to- 



induce the phenomenon, the 

 petioles relaxing and the pinnate 

 leaflets falling together like the 

 leaves of a book. This is also 

 said to be the case with several 

 of the Mimosas ; but the two- 

 species which are most common 

 in English stove-houses (M. 

 pudica, figs. 356, 357, and M. 

 sensitiva), though collapsing 

 readily at the slightest touch, 

 certainly do not exhibit such 

 FIG. 362. HEDGEHOG VARIETY OF HOLLY. extreme sensibility in this 



