376 THE HIGHER DICOTYLEDONS. 



In all cases the head is surrounded by an envelope (invo- 

 lucre) of bracts, which may be in one or two circles, or spirally 

 arranged and numerous, generally free but sometimes joined, 

 and which in some cases perform " sleep "-movements or close 

 up on being moistened. The inv. -bracts are usually green 

 and therefore carry on assimilation, but they serve mainly to 

 protect the young flowers and, later, the developing fruits, so 

 that they perform for the Composite flower-head the same 

 functions that in most other plants are performed by the calyx 

 of the individual flower. When spiny, the involucre protects 

 the flowers against browsing animals, and when the spines are 

 hooked (e.g. Burdock), it serves for animal dispersal of the 

 "seeds" (akenes). 



Since the ordinary functions of the calyx are transferred to 

 the involucre, the sepals of the individual flowers are rendered 

 unnecessary in this respect, and are often represented only by 

 a few small scales (free, or joined to form a collar) or bristles, 

 but in many cases it forms a pappus of long hairs which may 

 be sessile or raised, after fertilisation of the flower, on a long 

 stalk. The pappus-hairs are either rigid or silky, and either 

 simple (unbranched) or bearing minute knobs or secondary 

 hairs (feathery pappus) ; they are usually hygroscopic, 

 spreading out like a parachute in dry air and forming a most 

 effective means of wind-dispersal, and also helping to loosen 

 the fruits and detach them from the receptacle. 



In some cases, e.g. Bidens (Bur Marigold), the pappus 

 consists of barbed bristles, serving for animal-dispersal. In 

 Composites without a pappus the receptacle often contracts 

 in drying, so as to loosen and even jerk out the fruits 

 (Sunflower, etc.), or becomes conical (Daisy, etc.), so that 

 the fruits may be more readily carried away by wind. 



The common receptacle which carries the flowers the 

 enlarged end of the flowering axis is generally flat or rather 

 convex, and is either naked or bears bracts corresponding to the 

 individual flowers. The corolla is either tubular (generally 

 having a narrow lower part and an expanded upper part with 

 five lobes) or zygomorphic (the tubular corolla is generally 

 symmetrical, but sometimes not strictly so e.g. in Corn- 

 flower). Of the zygomorphic corolla there are several types. 

 Mutisia. In Dandelion and its allies (Hawkweeds, etc.) the 

 mouth of the corolla is drawn out, on the outer side of the 



