42 GENERAL BOTANY 



quickly moves upward again to the apogeotropic position (A), 

 which is favorable to the distribution of the seeds. Each seed, 

 thus exposed to air currents, is furnished with a parachute 

 apparatus composed of fine hairs which grow out from the 

 end of a long, stalklike beak projecting from the seed. This 

 hairy parachute is also sensitive to moisture, closing up in 

 wet weather and opening out on dry days, when the seeds 

 may more easily be disseminated. The envelopes of the entire 

 flower cluster are also extremely sensitive to light, moisture, 

 and temperature and assume a closed or an open position dur- 

 ing different stages in the development of the flower cluster, as 

 may be seen in Fig. 24. The flower clusters are also closed at 

 night and on cold days, and open in warm, bright weather. 



We see, therefore, in the dandelion a remarkable variety of 

 movements, most of which appear to be direct responses of 

 the various organs and their parts to stimuli induced by 

 gravity, light, moisture, and temperature. By means of these 

 responses the organism is not only able to adjust its organs 

 to the daily fluctuations of the environmental forces and con- 

 ditions but can also adapt its responses to the needs of par- 

 ticular organs at different stages in their development. Thus, 

 the opening and closing of the flower and fruit heads may be 

 a daily response and -adaptation, but the various positions 

 assumed by the scapes extend over many days and even weeks 

 and are determined by some unknown coordination between the 

 state or condition of the organ and its power of response to 

 gravity. Many other instances of similar adjustments of floral 

 parts to environmental conditions might be mentioned, but they 

 are all similar, in their general nature, to that of the dandelion 

 and need not be considered here. 



Trees and shrubs. In trees and shrubs, as we have already 

 learned, the ultimate form is assumed as a result of the general 

 plan of the plant body combined with adjusting movements. 

 The cohelike aspect of a pine tree, for example, is an excellent 

 illustration of the truth of the above statement. In the pine 

 (Fig. 10, 5) the main stem remains apogeotropic and continues its 

 vigorous growth during the life of the tree. The lateral branches, 



