MORPHOLOGY OF HIGHER PLANTS. 395 



of Populus the single flowers are pedicelled, and hence are actually 

 racemose rather than spicate inflorescences. 



In the head and the umbel the main inflorescential axis is 

 exceedingly short and the innermost flowers are often destitute 

 of bracts, in contrast with the external, which are frequently 

 provided with bracts that are of quite considerable size. Sterile 

 bracts also occur in these two types, and are called involucral 

 leaves, as in Cornus florida, where they are white or pink. There 

 is also a difference in sex of the outer and inner flowers. While 

 the head occurs as typical inflorescence in the Compositae, it also 

 exists in some of the Umbelliferae. 



The flowers of the Composite are borne on a common torus, 

 known as the disk, which is subtended by one or more circles of 

 bracts, these constituting an INVOLUCRE. The flowers are of two, 

 kinds, and they receive different names because of their form and, 

 position. Those situated near the margin of the disk are known 

 as RAY-FLOWERS, and because they possess more or less strap- 

 shaped corollas are also known as LIGULATE FLOWERS. Those 

 occupying the central portion of the disk are known as DISK- 

 FLOWERS, or as TUBULAR FLOWERS because of the tubular shape of 

 the corolla. Most of the Compositae possess both ligulate and 

 tubular flowers, as Arnica, Matricaria (Fig. 228), the common 

 daisy, etc. But some of the members of the family have only 

 ligulate flowers, as chicory and dandelion, and a relatively few 

 have only tubular flowers. 



Two types of definite inflorescence are distinguished : ( I ) 

 the DIBRACHIOUS (bifurcate) CYME in which the inflorescence 

 represents a series of very regularly arranged lateral axes, one 

 on each side of the terminal or median flower, as in the Caryo- 

 phyllaceae; and (2) the MONOBRACHIOUS (simple) CYME, of which 

 there are several modifications, but common to all of them is the 

 development of only one lateral branch to each terminal flower. 

 In the SCORPIOID cyme the lateral axes are arranged alternately 

 to the right and left, while in the HELICOID cyme the lateral axes 

 are all on the same side of the main axis, as in Hypericum. The 

 so-called flower cluster is a cymose inflorescence of either the 

 definite or indefinite type in which the flowers are almost sessile 

 or very short pedicelled, as in Chenopodium, Juncus, etc. Some j 



