136 



STRUCTURAL BOTANT. 



[116, 117. 



Phlox, Dodecatheon, and generally, the involucre is 

 green, but sometimes colored and petaloid, as in Dog- 

 wood and Euphorbia. Situated at the base of a com- 

 pound umbel, it is called a general involucre ; at the 

 base of a partial umbel it is a partial involucre or 

 involucel, both of which are seen in the Umbelliferse. 



433, Helianthus grosse-serratus I, involucre ; r, rays, or ligulate flowers. 434, One of the disk-flowers 

 with its chaff-scale (bract). 435, Acorn of Moss-cup Oak (Q. macrophylla). 436, Poa pratensis /, spikelet 

 entire ; g, glumes separated ; e, a flower separated, displaying the two pales, three stamens, and two styles. 



348. In the Compositee, where the flowers are 

 crowded upon a common torus, forming what is called 

 a compound flower, an involucre composed of many 

 imbricated scales (bracts) surrounds them as a calyx 

 surrounds a simple flower. The chaff also upon the 

 torus are bracts to which each floret is axillary (434). 



349. In the Grasses, the bracts subsist under the 

 general name of chaff. At the base of each spikelet 

 (436) of flowers we find two bracts the Glumes. At 

 the base of each separate flower in the spikelet are 

 also two bractlets the Pales enveloping as a calyx 

 the three stamens and two styles (c). 



350. The cup of the Acorn is another example of 

 involucre, composed of many scale-like bractlets. So, 

 also, perhaps the burr of the Chestnut, etc. 





