[Chap. XXXII FLOWERS, FRUITS, AND SEEDS 



355 



Fig. 153. Flower, fruit, and seeds of the garden pea. A, the flower; B, the 

 five petals; C, Hower with petals removed so that the stamen tube which sur- 

 rounds the pistil may be seen; D, arrangement of ovules as seen in a longitudinal 

 section of the pistil; E, F, and G, stages in the development of the fruit from 

 the pistil and of the seeds from ovules; H, a young ovule enlarged; I, enlarged 

 cross section of a young fruit to show that it is composed of one carpel and that 

 the ovules are borne on the infolded margins of the carpel. 



from separate primordia. The growing carpels, however, are united and 

 a compound pistil develops from them. 



A pistil that develops from one carpel, as in beans and peas, is a 

 simple pistil. In the flowers of some plants, such as the buttercup, the 

 carpels do not unite, and manv simple pistils develop separately in the 

 same flower. 



Still other plants have flowers in which the calyx and corolla are 

 tubular because the sepals and petals are united in a tube as they develop. 



Such unions of the parts of a flower take place between flower pri- 

 mordia, and the tubular structure develops thereafter as a unit. The 

 calyx of the flower of peas, for example, begins as five separate sepal 

 primordia near the apex of the receptacle. Shortly, however, later growth 

 at the bases of these primordia results in a complete collar of tissue. 

 Continued growth of this collar and of the lobe of each sepal results in 

 a cup-shaped calyx tube with five lobes at its margin. The ten stamen 



