648 



BOTANY OF THE LIVING PLANT 



Each head is surrounded by a common involucre of protective bracts. The 

 whole head is equivalent biologically to a single flower, and behaves as such, 



.."> 



Fig. 499. 



Floral diagram for Compositae. 

 (After Eichler.) 



Fig. 498. 

 Inflorescence of Daisy : a capitulum. (After Figuier.) 



though morphologically it is a closely packed, spicate inflorescence. The 

 Dandelion and Daisy are familiar examples (Fig. 498). The structure of the 



individual flower is already known in the case of 

 the Sunflower, by the study of its development 



..-*.-.^ (Chapter XIV., p. 273, Fig. 198). Each flower is 



there seen in the normal position, i.e. in the axil 

 of a bract ; it consists of 5 petals, 5 stamens, and 

 2 carpels. The transverse section of the flower 

 approaching maturity shows these parts arranged 

 as in a floral diagram. The odd petal is anterior ; 

 the stamens alternate with the petals, and the 

 carpels are antero-posterior (Fig. 198, viii.). The 

 ovary is inferior, unilocular, and contains one ovule. 

 This structure is fundamental for all Compositae 

 (Fig. 499). 

 As in similarly crowded inflorescences (for in- 

 stance the cyathium of the Spurges, p. 629), the crowding brings with it 

 reduction of the individual flowers, but it does not go so far in the Compositae 

 as in the Spurges. The most usual modification is the reduction of the calyx, 

 its protective function having devolved upon the involucre. Sometimes it 

 is absent, as in the Daisy ; or it may be represented by two or three teeth, as 

 in Bidens (Fig. 251, E). But most frequently it is replaced by a number of 

 bristles composing what is called the " pappus," which serves as a means of 

 fruit-dispersal, taking the form of a " pappus " composed of bristles, which 

 spread like a parachute (Fig. 247, p. 326). The bract subtending each flower 

 is often abortive, as in the Dandelion and Daisy. The flowers themselves 

 though typically hermaphrodite are liable to become unisexual by abortion. 

 These are all features of reduction, following on the aggregation of the flowers 

 in the compact inflorescence. 



The flowers may develop in three different ways, though all are fundamen- 

 tally of the same construction, having the general formula, S, (5, or less, or o, 



or x), P. 5, A. 5, G. (2). The first type is radially symmetrical, with five equal 



» ' 



petals. This is probably the original type, and is characteristic of the florets 

 of the disc (Fig. 500, in., iv.). A second type is seen in the ray-florets (Fig. 

 500, v.), in which the corolla is tubular below, but the three anterior petals are 

 elongated into a long strap-shaped ray, as shown by the three distal teeth ; 

 the two obliquely posterior are reduced or absent. These ray-florets are 



