PHYSIOLOGICAL PROCESSES DURING FLOWERING 433 



The rapid growth occurrmg in opening flowers is manifested 

 not only in the general increase in size of the separate parts 

 of the flower but also in the peculiar growth movements generally 

 defined by the term "nastic movements/' or ''nasties." In 

 Chap. IV, one type of growth movements, viz., tropisms, has 

 already been discussed. As has been seen, these movements are 

 caused by the influence of some unilateral external factor, e.g., 

 light or gravity, acting in a definite direction. The growing 

 organs are curved in a definite direction in relation to this factor. 

 For instance, the tops of stems placed horizontally curve upward, 

 or growing seedlings curve in the direction of light. 



The term nasties is applied to such movements as are produced 

 by stimuli that have no definite direction, resulting in no par- 

 ticular orientation of the plant. A dorsiventral or bilaterally 

 symmetrical structure of the curving organ is indispensable for 

 movements of a nastic nature. Those involving changes in 

 turgor, otherwise called ''alternating movements," are of far 

 greater importance than in tropisms. 



Nyctinastic movements caused by succession of night and 

 day are very common. Many flowers open in the morning 

 and close for the night. Some leaves, particularly the compound 

 ones of papilionaceous plants, change their position during the 

 night and day. These movements occur with considerable 

 regularity, which led Linnaeus to attempt to construct a "floral 

 clock" by making a bed of plants in which the different flowers 

 would open and close at various hours of the morning and 

 evening. Movements of this nature are often said to be con- 

 nected with the "sleep of plants." This expression is inappro- 

 priate, as such nyctinastic movements have nothing in common 

 with the sleep of animals, which is a pecuhar condition of rest. 



Changes in temperature or the intensity of fight, or both, are 

 the actuating causes of these movements. The first feature is 

 concerned with thermonasty; the second, with photonasty. The 

 opening of flowers of the tulip or saffron. Crocus sativus, which 

 may be easily seen when these plants are carried from a cold 

 into a warm room, is a typical instance of thermonasty. Under 

 these conditions, the flowers will open in 3 to 5 mm. (Fig. 135). 

 The mechanism of this movement lies in the fact that with an 

 increase of temperature the growth of the inner surface, morpho- 

 logically the upper surface, overtakes the growth of the lower 



