SPECIAL FOttMS OF INFLORESCENCE. 117 



composed of many scale-like bractlets. So, also, perhaps the 

 burr of the Chestnut, etc. 



351. The forms of inflorescence are exceedingly various, but 

 may all be referred to two classes, as already indicated the 

 axillary, in which all the flowers arise from axillary buds ; the 

 terminal, in which all the flower-buds are terminal. 



352. Axillary inflorescence is called indefinite, because the 

 axis, being terminated by a leaf-bud, continues to grow on in- 

 definitely, developing bracts with their axillary flowers as it 

 grows. It is also called centripetal, because in the order of time 

 the blossoming commences with the circumference (or base) of 

 the inflorescence, and proceeds toward the central or terminal 

 bud, as in Hawthorn or Mustard. 



353. Terminal inflorescence, on the other hand, is definite, im- 

 plying that the growth of the axis as well as of each branch is 

 definitely arrested and cut short by a flower. It is also centrif- 

 ugal, because the blossoming commences with the central 

 flower and proceeds in order to the circumference, as in the 

 Sweet-William, Elder, Hydrangea. 



354. Both kinds of inflorescence are occasionally combined in the same plant, where 

 the general system may be distinguished from the partial clusters which compose it. 

 Thus in the Compositae, while the florets of each head open centripetally, the general 

 inflorescence is centrifugal, that is, the terminal head is developed before the lateral 

 ones. But in the Labiatie the partial clusters (verticillasters) open centrifugally, while the 

 general inflorescence is indefinite, proceeding from the base upward. 



Review. 341. Define inflorescence. Metamorphosis of buds. 342. Position of buds. 

 What may a single bud develop ? 343. What is a peduncle ? May it bear leaves ? What 

 the pedicels ? Rachis? When is a flower sessile? 344. How does a scape differ from 

 peduncle? Where is the torus ? 345. What are bracts ? Bractlets ? How are the bracts 

 inCrucifers? What of color? 346. Define spathe. Illustrate. 347. Define involucre 

 Involucel. 348. Describe a (so-called) compound flower. What is the chaff? 349. In 

 Grasses, what are the glumes ? The pales ? 350. What is the nature of the cup in Oak ? 

 Why is axillary infl. called indefinite ? Why centripetal ? Why is terminal infl. definite ? 

 \\ hy centrifugal ? How are both combined in Compositae ? 



CHAPTER XXVI. 



SPECIAL FORMS OF INFLORESCENCE. 



355. Of centripetal or axillary inflorescence the prin- 

 cipal varieties are the spike, spadix, catkin, raceme, corymb, 

 umbel, panicle, thyrse, head. The spike is a long rachis with 



