298 MODIFIED CIKCUMNUTATION. CHAP. VI 



ayctitropic, unless it has been acquired for the sake of 

 lessening radiation ; but this could be discovered only 

 by a long series of experiments, showing that the 

 leaves of each species suffered from this cause, if pro- 

 vented from sleeping. We must therefore take an 

 arbitrary limit. If a cotyledon or leaf is inclined at 

 60 above or beneath the horizon, it exposes to the 

 zenith about one-half of its area ; consequently the 

 intensity of its radiation will be lessened by about 

 half, compared with what it would have been if the 

 cotyledon or leaf had remained horizontal. This 

 degree of diminution certainly would make a great 

 difference to a plant having a tender constitution. 

 We will therefore speak of a cotyledon and hereafter 

 of a leaf as sleeping, only when it rises at night to 

 an angle of about 60, or to a still higher angle, above 

 the horizon, or sinks beneath it to the same amount. 

 Not but that a lesser diminution of radiation may be 

 advantageous to a plant, as in the case of Datura 

 stramonium, the cotyledons of which rose from 31 at 

 noon to 55 at night above the horizon. The Swedish 

 turnip may profit by the area of its leaves being 

 reduced at night by about 30 per cent., as estimated 

 by M r. A. S. Wilson ; though in this case the angle 

 through which the leaves rose was not observed. On 

 the other hand, when the angular rise of cotyledons or 

 of leaA r es is small, such as less than 30, the diminution 

 of radiation is so slight that it probably is of no sig- 

 nificance to the plant in relation to radiation. For 

 instance, the cotyledons of Geranium Ibericum rose at 

 night to 27 above the horizon, and this would lessen 

 radiation by only 11 per cent. : those of Linutn Beren- 

 dieri rose to 33, and this would lessen radiation by 

 10 per cent. 



There are, however, some other sources of doubt witn 



