MORPHOLOGY OF ANGIOSPERMS. 139 



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 Horida where they are white or ])ink. 

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

 (see page 391). While the head occurs as typical inflorescence 

 in the Compositae, it also exists in some of the Umbelliferse. 



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- 

 phyllaceas; 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. Sometimes the inflorescences may be 

 decompound or complex, as in several Compositae, where the heads 

 may be arranged in cymes or racemes; or, as in the Gramineae, 

 where the spikelets, which are spikes, may be arranged in panicles, 

 i.e., branched racemes; or finally, as in Cryptotaenia (Unibel- 

 liferae), where the umbels are arranged in cymes. 



Pollination and Fertilization, Fertilization represents the 

 final stage in the work of the flower as a whole, and has already 

 been defined as the union of the egg-cell and a male nucleus. 

 Pollination may be considered to include the transferral of the 

 pollen grains from the anther to stigma and their subsequent 

 germination thereon, this latter process resulting in the produc- 

 tion of the male nuclei. Pollination thus represents but one series 

 of changes or processes which precede fertilization, for, while the 

 pollen grain is going through the various stages in development 

 which lead to the formation of the male nuclei, a series of coni- 

 plex changes are going on in the embryo-sac leading to the devel- 

 opment of the egg-cell. 



