THE ANGIOSPERMAE 



1161 



best known exceptions. Hairy flowers, like hairy leaves, are usually associ- 

 ated with xerophytic conditions, as in many Ericaceae in South Africa, 

 but this is not so in Hoya carnosa (Asclepiadaceae) (see Fig. 1798) which 

 is a tropical climber from S.E. Asia. Sepaloid and staminoid modifications 

 of the petals are widespread and have already been described; less com- 

 mon are those which are sepaloid in the middle and petaloid at the edges, 

 as in some degree occurs in several Liliaceae. The Compositae provide a 

 very interesting study in the modification of the corolla. The distinction 

 between the small, actinomorphic corollas 

 of the disc flowers and the large, unilate- 

 rally developed corollas of the ray flowers 

 is well known, but the form of the latter 

 is very variable, the " ray " or ligulate 

 portion being formed from two, three, 

 four or all five of the united petals. 

 Troll has shown that in the details of its 

 structure this ligulate portion often re- 

 produces, even in small details, features 

 characteristic of single petals, which he 

 regards as supporting his conception of 

 the overriding influence of the " flower 

 model " or pattern in shaping structures 

 of diverse morphological character. 



Petals are often diflterentiated into a 

 narrow basal part, called the claw, and a 

 flat, expanded part called the limb. The 

 petals of the Cruciferae afl^ord a good 

 example, the narrow claws of the four 

 petals standing vertically, so close to the 

 stamens and to each other that they make 

 a good substitute for a floral tube of the 

 Primula type. Some bizarre extremes of 

 this type of petal formation are to be seen 

 in the Sterculiaceae, the petal limbs 



appearing on the ends of long, delicate filaments, which gives the whole 

 structure a striking resemblance to a stamen. The Asclepiadaceae provide 

 numerous examples of the opposite modification, the apex of the petals 

 being here drawn out into long filaments, which may be many times as 

 long as the flowers (Fig. 1132). 



One of the most striking modifications of the normal corolla is the deve- 

 lopment of an inner ring of ligule-like appendages, the paracorolla, which 

 may be united into a membrane, like a belt, called the corona. Examples of 

 these structures are widespread, in many families, but they are best known 

 in the Amaryllidaceae, where the prominent corona of Narcissus, often 

 extended into a long, tubular " trumpet ", is familiar to all. 



Many morphological interpretations have been put upon this structure, 



Fig. 1 132. — Trichosacme latiatd. Petals 

 with long hairy appendages. {After 

 Velenovskw) 



