1 86 BIOLOGICAL LECTURES. 



{Oxalis stricta), if examined on a quiet and moderately cool 

 day when the temperature is 25° C, will show the trifoliate leaf 

 fully expanded, and the three leaflets so placed on their stalks 

 that their surfaces lie at right angles to the sun's rays ; i.c ., 

 they are not only heliotropic in that they grow toward the 

 light, but they are diaheliotropic in that they can, if need be, 

 so place their surfaces to the light as to receive the rays 

 at right angles. But if the temperature rise to 29°- 30° 

 C. in the shade, the leaflets will begin to fall down ; and if 

 a steady increase in temperature goes on, they will continue 

 to fall till at 33° C. they will incline downwards back to back. 

 This day-movement has long been known to occur in many 

 plants, and was regarded as a means by which the leaves 

 were screened from intense illumination ; but my colleague, 

 Prof. W. P. Wilson, considers it to be a heat-movement, or an 

 attempt on the part of the plant to screen itself from 

 intense solar thermal action. As connected with the varying 

 results of stimulation on different species afterward to be 

 described, it may here be noted that Oxa/is stricta shows 

 distinct heat-movement at 29° C. ; Oxalis Dcppci at 31° C; 

 Oxalis dendroides at 33°-34° C. ; while the common sensitive 

 plant {Mimosa piidica) that has thin and apparently delicate 

 leaflets only becomes affected when the shade temperature 

 rises to 37° C. This movement, described by Darwin as a 

 paraheliotropic one, I propose to term ' parathermotropic' 



But as the sun's rays become tempered in the afternoon, or 

 as plants formerly exposed to the full heat of the rays get 

 sheltered by foliage, the flat, expanded condition is resumed. 

 Toward the approach of evening the leaflets again begin to 

 fall, and by 8.15 or 8.30 during midsummer have taken up the 

 same position that they had during the heat of the day. This 

 night-sleep, or nyctitropism, has, with every show of reason, 

 been viewed by Darwin and others as a protection against too 

 rapid radiation of heat and reduction of temperature in the 

 tissues. We would emphasize it, then, that alike during the 

 parathermotropic and nyctitropic states the leaflets of Oxalis 

 stricta are similarly placed. The nyctitropic position is 

 retained till the following morning, and in some plants, at 



