CHAPTER XXXII.* 



PROTHALLIUM AND SEXUAL ORGANS OF 

 FLOWERING PLANTS. 



333. The stamens and pistils are not the sexual organs. 



Before the sexual organs and sexual processes in plants were 

 properly understood it was customary for botanists to speak 

 of the stamens and pistils of flowering plants as the sexual 

 organs. Some of the early botanists, a century ago, found 

 that in many plants the seed would not form unless first the 

 pollen from the stamens came to be deposited on the stigma of 

 the pistil. A little further study showed that the pollen 

 germinated on the stigma and formed a tube which made its 

 way down through the pistil and into the ovule. 



This process, including the deposition of the pollen on the 

 stigma was supposed to be fertilization, the stamen was looked 

 on as the male sexual organ, and the pistil as the female sexual 

 organ. We have found out, however, by further study, and 

 especially by a comparison of the flowering plants and the lower 

 plants, that the stamens and pistils are not the sexual organs of 

 the flower. 



334. The stamens and pistils are spore-bearing leaves. The 

 stamen is the spore-bearing leaf, and the pollen grains are not un- 

 like spores; in fact they are the small spores of the angiosperms. 

 The pistil is also a spore-bearing leaf, the ovule the sporangium, 

 which contains the large spore called an embryo sac. In the 

 ferns we know that the spore germinates and produces the green 

 heart-shaped prothallium. The prothallium bears the sexual 



* This chapter is for reading and reference, but if the teacher desires to 

 give demonstrations of the germinating pollen grain, and of the embryo sac, 

 the following memorandum on material will be found of assistance. 



203 



