MOVEMENT 365 



two have been especially studied,— those produced by light and 

 those by temperature. The movements due to light are commonly 

 called " sleep," nyctitropic, photeolic, and photonastic move- 

 ments. The dandelion {Taraxacum) and many composite flower 

 heads open during the day and close at night; night-flowering 

 plants such as Silene nodiflora, the evening primroses, etc., be- 

 have in the opposite fashion. This arrangement keeps the flower 

 open when its pollinating insects are about and protects them 

 against rain, dew, or wind at other times. 



While the turgor changes are here seen to be of advantage, it 

 is harder to explain the movements of many legume leaves which 

 drop at night and fold together. In this night or " sleep' ' posi- 

 tion, as seen in the clover and oxalis, the petiole curves either up 

 or down and the blades are generally placed with one definite 

 face outwards. The night position is often such that the stomatal 

 surface is directed inwards, and it has been suggested that this 

 arrangement serves to check radiation and the formation of dew 

 in the stomata, thus favoring transpiration in the morning. This 

 explanation is based upon the assumption that transpiration is 

 an advantage, which is very doubtful (Chap. XX). If these move- 

 ments have any advantage, which is by no means certain, it would 

 probably be in checking transpiration during the night and thus in- 

 creasing the turgor for the following day's work. Since the changes 

 in light are the most pronounced at dawn and twilight, the phote- 

 olic movements are easily observed at that time. In Oxalis the 

 temperature seems to play a role but humidity is of little impor- 

 tance; while in the legume leaflets, humidity is important (Gates, 

 1916). Such movements, produced by light and humidity com- 

 bined, have been called xerophotic. Zimmerman (1929) has noted 

 turgor differences of seven atmospheres between the upper and 

 lower sides of the leaf joint (pulvinus) in such leaves. 



Thermonasty is shown by the perianth segments of many flowers, 

 especially in members of the Liliacese (Crocus, Tulipa, Ornitho- 

 galum, and Colchicum), whose petals, during a rise of tempera- 

 ture, suddenly grow larger on the inner side of the base. The 

 result is the opening of the flower. Crocus and tulip when brought 

 in from out of doors to a temperature 15° C. warmer will open in 

 three minutes. The crocus and Gentiana ciliata are sensitive to a 

 difference of 0.2-0.5° while the tulip, only slightly less sensitive, 

 responds to a change of 1-3° C. When the temperature is lowered, 



