118 PLANT PHYSIOLOGY AND ECOLOGY 



amount of pollen produced decreases as the method of pollination 

 becomes more and more perfected. Moreover, accessory staminate 

 flowers disappear, and the number of flowers is often greatly 

 raluced also. In practically all zygomorphic flowers, i.e., those 

 that apparently owe their irregular form to insect-pollination, 

 such as the mints, snapdragon, orchids, etc., the number of 

 stamens is regularly reduced to four or two. In certain orchids 

 the number of pollen masses, or pollinia, is reduced to two or one, 

 so certain has pollination become. 



Experiment 38, Amount of pollen. Make a comparison of various 

 species with respect to the number of flowers, and the number of stamens 

 in each flower Estimate the number of pollen grains in an anther 

 and determine the total pollen production of a plant of each type. 



141. Protection of pollen. Flowers protect their pollen against 

 injury from rain or dew by means of the most diverse modifica- 

 tions. In many instances the protection afforded is secondary, 

 the structure or modification having been developed chiefly for 

 other reasons. The most striking devices, especially those involv- 

 ing a movement of the plant or its parts, serve primarily for })ro- 

 tection. A number of plants, particularly wind-pollinated ones, 

 seem to have no protection against injurious moisture. In many 

 of these it will probably be found that the protection, though 

 obscure, is effective. The period of flowering and the time of 

 flowering, when more thoroughly investigated and understood, 

 will suffice to explain why some flowers seem unprotected. It 

 is probable that plants in which the pollen is not protected from 

 harm in some way do not occur, since the existence of a species 

 is proof of such protection. In some plants indifference to the 

 effect of rain or dew apparently constitutes an ample protection. 



The damage that results from wetting is not the same for 

 every plant. In some, moisture causes premature germination; 

 in the majority it interferes seriously with the transfer of the 

 pollen. In all these cases the protection afforded the pollen serves 

 also for the nectar. In some instances it is possible that the 

 device has really been developed for the latter purpose. 



Pollen must be protected against dryness as well as against 

 moisture. This is usually accomplished so effectually by the 

 structure of the grain itself that other modifications for this purpose 

 are obscure, if not altogether lacking. In the case of desert plants 



