122 PLANT PHYSIOLOGY AND ECOLOGY 



Ephemeral flowers, e.g., Tradescantia, Portidaca, Onagra, etc., 

 open only under favorable conditions, and wilt after a few hours, 

 thus reducing the chance of injury to a minimum. The blooming 

 of flowers at particular times during the growing period doubtless 

 has some connection with the presence or absence of definite pro- 

 tective devices. JMany species have formed the habit of flowering 

 at the season in which there is least danger from rains. Seasonal 

 protection sometimes occurs along with morphological or mechan- 

 ical devices, producing a double protection. 



Experiment 39. Protection of pollen. Make careful field observa- 

 tions upon the spring flowers, and note whether the pollen is sheltered 

 or not. Study and sketch some of the ways in which protection is 

 brought about, and arrange the species in groups according to the 

 method employed. 



145. Disposition of the stamens and pistils. Since the pollen 

 must be transferred from the anthers to the stigma in some way 

 in order to insure fertilization, the relative position and develop- 

 ment of the stamens and pistils become matters of the greatest 

 importance. They not only affect the method of transfer, but 

 they also determine the kind of fertilization that results. The 

 arrangement by which stamens and pistils occur in different 

 flowers is termed diclinism, and plants which possess staminate 

 and pistillate flowers are said to be didinic. Diclinic species 

 are monoecious w^hen the staminate and pistillate flowers occur 

 upon the same plant, and dioecious when the staminate flowers 

 grow upon one plant and the pistillate ones upon another. Plants 

 with stamens and pistils in the same flower are said to be mouo- 

 clinic. The presence of monoclinic and diclinic flowers in the 

 same species is called 'polygamy. In monoclinic or perfect flowers 

 the rule is for stamens and stigmas to mature at different times, 

 in order to increase the chance of cross-pollination. This condition 

 is termed dichogamy. Dichogamous flowers are protandrous when 

 the anthers shed their pollen before the stigma becomes receptive, 

 and protogynous when the stigma matures first. The stigma has 

 been termed short-lived w^hen it is receptive for a short time, 

 and long-lived when it remains receptive, i.e., capable of caus- 

 ing pollen to germinate, for several days or more. When the 

 anthers and stigma mature at the same time, the flow'er is termed 

 homogamous. 



