REPRODUCTION IN PLANTS 



267 



with the parts of adjacent whorls. Thus several carpels may fuse to form 

 a compound ovary, or the stamens may join into a solid ring, or the petals 

 may unite into a tubular structure. Frequently the stamens appear to 

 arise from the inner surface of a tubular corolla, their bases having fused 

 with those of the petals. The pistil may be attached to the receptacle 

 merely at the base, so that the ovary is exposed and is said to be superior; 

 or the sides of the receptacle may grow up around the ovary and fuse with 

 its walls, in which case the ovary is said to be inferior. 



Arrangement. Besides the differences that exist in the flowers them- 

 selves, their arrangement on the 

 axis (inflorescence) varies greatly 

 from plant to plant. Sometimes a 

 single flower is borne at the end 

 of the unbranched peduncle. Often 

 the peduncle does not produce a 

 flower at its tip but sends off 

 lateral pedicels, each bearing a 

 flower. If the pedicels and 

 peduncle are both long, this makes 

 a loose cluster of flowers called a 

 raceme, as in wild plum; if the 

 pedicels are very short or if the 

 flowers are sessile, we have a spike, 

 as in plantain, or various modified 

 types of spikes, such as the spikelet 

 of grasses, the catkin of willows, 

 the spadix of white arum and 

 Jack-in-the-pulpit, and the scaly 

 strobilus of hops. 



When, in this type of inflores- 

 cence, the branching is continued, 

 various kinds of compound 

 racemes and spikes are produced. One very common form is the 

 flower cluster (umbel) of the Umbelliferae, in which the axis is very 

 short and the branches and pedicels are long and radiating. An- 

 other common type of flower arrangement is the cyme, produced when the 

 central axis terminates in a flower but sends off side branches, which 

 themselves end in flowers; these may continue the branching process, 

 giving rise to compound cymes. 



Finally, we have in one very large group of plants, the composites (of 

 which daisies and sunflowers are examples), flower clusters consisting of 

 dozens or hundreds of small individual flowers borne crowded together 

 on the surface of a greatly enlarged and conical or flattened receptacle. 



Fig. 17.13. Part of the head of the shasta 

 daisy (Chrysanthemum maximum, family 

 Compositae), showing perfect disk flowers 

 with reduced corollas (left) and one of the 

 imperfect ray flowers with strap-shaped, 

 petal-like corollas (right). (Photo by Prof. 

 E. B. Mains.) 



