142 MOVEMENTS OF CURVATURE 



an immotile condition l . This occurs in Mimosa pudica after three to six 

 days' darkness, and the pulvini of other plants behave similarly. Very 

 feeble illumination induces rigor in the leaves of M\ imosa pudica, but suffices 

 to keep the pulvini of the shade-loving Oxalis acetosella in a phototonic 

 condition. According to Jost, the seismonic irritability disappears first in 

 some cases, but usually the photonastic irritability is lost first, while the 

 rigor is usually, but not always, more rapidly induced at high temperatures 2 . 

 The rigor of the pulvini of foliage-leaves is apparently the result of a patho- 

 logical condition induced by continued darkness, and ultimately leading to 

 death 3 . Since the leaves are also injured when exposed to light in air 

 deprived of carbon dioxide in which photosynthesis is reduced to a very 

 low ebb 4 , the pathological condition induced by darkness is probably the 

 result of the leaf being unable to perform its normal function. The rigor 

 does not appear to be due to any deficiency of food or to the lack of any 

 autoassimilatory products, for it is produced without any fall of turgor 5 , 

 and in some cases when the leaves are abundantly provided with food 6 , as 

 also are accompanying pathological changes, such as the alteration in colour 

 of the chloroplastids and the temporary or permanent loss of the power of 

 photosynthesis 7 . It is not surprising that a leaf developed in light may 

 be unable to accommodate itself to darkness, whereas under special circum- 

 stances a leaf may develop to a considerable size and acquire irritability 

 in continuous darkness. 



The experiments with coloured light lack critical precision, but, as far 

 as they go, seem to indicate that phototonus is maintained by the more 

 refrangible as well as by the less refrangible halves of the spectrum 8 . 

 Although the blue and violet rays exercise a stronger photonastic action, 

 nevertheless the red and yellow rays are able to induce the sleep-movements 

 of leaves. The movements, however, begin earlier, and take place more 

 rapidly in blue light than in red, just as when the effects of strong and of 

 feeble illumination are compared. Similar differences are shown by the 

 nutation movements of chlorophyllous and non-chlorophyllous organs, 

 while flowers open less in red light or under feeble white light than when 

 exposed to the blue rays . 



1 Sachs, Flora, 1863, p. 499, and the literature there given; Jost, Jahrb. f. vviss. Bot., 1895, 

 Bd. xxvn, p. 457. 



2 Jost, 1. c., pp. 465, 469. 



8 Pfeffer, Period. Bewegungen, 1875, p. 64; Jost, I.e., p. 457. 

 * Ewart, Journ. Linn. Soc., Vol. XXXI, 1897, p. 569. 

 : ' Pfeffer, 1. c., p. 68. 

 8 Pfeffer, 1. c., p. 64. 



7 Ewart, I.e., pp. 568, 570. 



8 Daubeny, Phil. Trans., 1836, I, p. 519; Bert, Mem. de 1'Acad. de Bordeaux, 1871, p. 28 of 

 reprint ; W. P. Wilson, Contrib. from the Bot. Lab. of Pennsylvania, 1892, Vol. I, p. 71 ; Macfarlane, 

 Bot. Centralbl., 1895, Bd. LXI, p. 136. 



9 Hansgirg, Physiol. u. phycophytol. Unters., 1893, p. 60. 



