Rural School Leaflet 1287 



There is still another form of inflorescence in which the flowers are 

 sessile. This is found in sunflowers and daisies and is called a head. The 

 arrangement of clover flowers also is called a head. A sunflower head is 

 called a composite head. In it, as is shown in Fig. 13, we find two kinds 

 of flowers. The center flowers are short and densely massed together, 

 and are called disk flowers. The outer flowers (a) are usually conspicuous 

 and are called ray flowers. Outside of the ray flowers we find a series 

 of green bracts, which apparently bind together the flowers of the head. 

 This series of structures is called the involucre, and its parts are the bracts 

 of the involucre. The broad structure on which the flowers are borne 

 (d) is called the receptacle. A composite head may be composed en- 

 tirely of ray flowers or the ray flowers may be entirely absent. 



The remaining forms of inflorescence have the flowers borne on pedicels 

 instead of being sessile. One of the simplest forms is the raceme, Fig. 7. 

 In this case the flowers are borne on pedicels arranged along a common 

 axis, as in a spike, the difference being that here the flowers are not sessile. 

 In a raceme the lower flowers are the older, and usually the larger. 

 Growth continues for an indefinite time and is said to be indeterminate. 

 A cyme differs from a raceme in that the terminal flowers are the older 

 and larger. New flowers appear below the old ones, and growth in length 

 ceases with the first flowers. A growth of this type is called a determinate 

 growth. A cyme is shown diagrammatically in Fig. 8. 



In a cyme and a raceme the pedicels on which the flowers are borne 

 are shorter than the main axis, and the flowers are arranged in a sort of 

 pyramid. A form of inflorescence exists, however, in which the flower 

 cluster is flat-topped, as in the common yarrow. This form of inflores- 

 cence is called a corymb. A corymb may be a determinate or an indeter- 

 minate form of inflorescence, in the general use of the term. In Fig. 9 

 is shown a determinate corymb. 



In the forms previously described, the flowers have arisen from different 

 points on the same or different axes. A form is found rather commonly, 

 however, in which all the flower axes arise from the same point. Such 

 an inflorescence is called an umbel and is shown in Fig. 12. 



Various modifications of the above forms exist. A raceme, cyme, 

 corymb, or umbel may be compound; that is, the axillary branches of 

 an inflorescence may in turn be divided into the given type. A compound 

 raceme of a loose, irregular nature is called a -panicle. 



Leaves are simple or compound. A simple leaf has but one blade. 

 A compound leaf has more than one blade. The various types of leaves 

 are illustrated on page 1284. There are other types, but most of the 

 common forms are shown. Compound leaves are of two types, pinnately 

 compound and palmately compound, as shown in Figs. 10 and 11. A pin- 



