ADJUSTMENT TO TEMPERATURE 



121 



many erect ones by the bending or drooping of the flower-stalk. 

 This is notably the case in Campanula rotundijolia and to a less 

 extent in C. aparinoides, in which the bud is erect but the flower 

 is drooping. In Oxalis, Anemone, etc., the bending takes place 

 more quickly and lasts overnight only, or, more rarely, throughout 

 cloudy or rainy days. In some cases the entire flower cluster 



a 



Fig. 30. Protection of pollen in the California poppy, EschschoUzm cali- 

 fornica, by the rolling of the petals in wet weather (b). (After Kerner.) 



droops, as in certain geraniums, umbellifers, and composites. In 

 some radiate and ligulate flowers of the last family, protection is 

 afforded by the upward or inward movement of the ligules. In 

 radiate flowers, such as the asters, the protection of the disk 

 florets by the rays is only partial, l^ut in species of the Ligidiflone 

 each floret is protected by its own ligule and by the longer ligules 

 of the florets nearer the margin. 



The movements of the flower and its parts are usually referred 

 to as anthotropism. The mechanism by w^hich the movement is 

 produced is not well understood in most cases. The opening and 

 closing of the flower, as well as the bending of the rays in com- 

 posite heads, is now definitely known to be caused by variations 

 in temperature, and not in light, as is commonly supposed. The 

 precise nature of the response to heat is not known, but it seems 

 to be a direct reaction of the protoplasm. The bending of flower- 

 stalks may be due to more rapid growth upon one side than the 

 other, or to the attraction exerted by gravity. 



144, Seasonal protection. The seasonal protection exhibited 

 by many flowers is concerned with the time or period of flowering. 

 This is shown in plants such as Ipoma:a, Taraxacum, etc., which open 

 their flowers in bright sunshine and close them u]ion the apjiroach 

 of rain or at nightfall, thus effectively sheltering the ijollcn. 



