VARIATIONS IN STRUCTURE 



285 



ranged on the stem; they are not arranged in definite 

 rings or whorls. The white water lily illustrates this point 

 very well. In it we find the many petals spirally arranged. 

 We also find 

 parts which 

 are partly 

 petal and partly stamen. 

 (See Figure 108.) The 

 stamens and carpels as 

 well as the petals are, as 

 to number, numerous 

 and indefinite. This 

 latter characteristic is 

 considered more primi- 

 tive than the low and 

 regular number of parts 

 which characterize most 

 of our familiar flowers. 

 In all these respects the 

 water lily is regarded a 

 rather primitive type of 

 flower. 



Petals and sepals are 

 either separate or more 

 or less united. Flowers 

 with these parts sepa- 

 rate are considered more 

 primitive than those in which they are united. This ap- 

 plies especially to the petals. Some flowers have united 

 petals, but separate sepals. The bluebell, the phlox, the 

 morning-glory, and the petunia are common plants whose 

 petals are united. (See Figures 93 B, 107, and in.) Flowers 



Fig. 105. — Flowers of the American aspen 

 (Populus tremuloides) ; sometimes called the 

 trembling aspen. It is dioecious. A shows 

 a catkin composed of staminate flowers. 

 B, pistillate catkin. C, a single pistillate 

 flower with the characteristic scale which 

 accompanies it. D, the staminate flower 

 and scale. 



