152 



ORGANIC EVOLUTION 



All are familiar with the general structure of plant 

 blossoms (Plate 86). Within the brightly colored floral 

 leaves are found two sets of reproductive organs: an inner 

 set, female, called carpels, or when united as they com- 



FlG. 44. Fertilization in the rock-rose (Helianthemum marifolium). [After KEENER 



i. A single flower, natural size. 2. A flower, stripped of its sepals and petals, showing the 

 pistil in longitudinal section. The pollen grains are seen upon the stigma, and their tubes are 

 seen passing down the stalk of the pistil to reach the ovules. The tubes are indicated erroneously 

 as going direct to the openings at the tips of the ovules ; actually they follow a more devious 

 course, first down the inside wall of the chamber of the pistil and then up to reach the apertures 

 in the ovules ; ov. = ovule, stg. = stigma. 3. A more enlarged drawing of the tip of the pistil, show- 

 ing the pollen grains and the sprouting pollen tubes. 4. A dry pollen grain. 5. A moistened 

 pollen grain developing its tube. 6. An ovule, showing the opening at its tip through which the 

 pollen tube enters to effect fertilization. 



monly are, together composing the pistil ; and an outer 

 whorl of stamens, or male organs. The ovules, or imma- 

 ture seeds, are formed within the pistil (Fig. 44, ov), while 

 the pollen, by which the ovules are to be fertilized, is 

 formed in the anthers at the tips of the stamens. To 



