1 70 GROWTH AND WORK OF PLANTS 



process is going on continually, but the greater amount of change 

 appears to go on during late afternoon, during the night, and 

 in the early morning. 



273. Ray flowers. The ray flowers have a rudimentary 

 ovary with which the calyx is consolidated, but the pistil is other- 

 wise undeveloped and stamens are wanting. Such flowers are 

 sterile, and are often called neuters. The corolla, however, is 

 very conspicuous. The petals are united by their edges in such 

 a way as to form a long, broad, leaf-like organ, except at the 

 base where they form a short tube. The ray flowers of the 

 sunflower do not show well the number of divisions of the corolla. 

 But the ray flowers of some other composites do, as in the core- 

 opsis, etc., where the end is five-toothed. In many composites 

 the ray flowers are pistillate, and therefore develop seed like the 

 disk flowers, as in the ox-eye daisy, or white weed. In the dan- 

 delion, chicory, etc., all of the flowers are strap-shaped (like the 

 ray flowers) and are fertile. For composites of economic impor- 

 tance see Chapter XXXVIII. 



274. The composite flower shows the highest degree of spe- 

 cialization, by the union of parts and the massing of the flowers, 

 of all the seed plants. They are thus believed to represent the 

 highest stage of evolution in plants. The buttercup flower is 

 much more simple in its structure, for all the parts are separate 

 and distinct, and some of them formed in large numbers. There 

 are other dicotyledons, however, in which the flowers are still more 

 simple in structure. Some of the flowers which we have studied 

 show different steps in the consolidation of parts of the flower. 

 In the evening primrose the sepals are united among themselves 

 (gamosepalous), and are adnate with the ovary, while the petals 

 and stamens are seated on the calyx tube instead of on the recep- 

 tacle of the flower. In the "butter and eggs," not only is the 

 calyx formed by the united sepals, but the corolla is formed by 

 the united petals (gamopetalous). In the morning glory, calyx, 

 corolla, and pistils all show a union of their parts, i.e., the calyx 

 is gamosepalous, the corolla is gamopetalous, and the pistil is 

 compound. But they are all free, except the stamens, which are 



