1552 



A TEXTBOOK OF THEORETICAL BOTANY 



Perianth members, usually sepals but occasionally petals, may be 

 retained as wings in the fruiting stage and often develop considerably in 

 size. The most striking and one of the best known examples of sepal 

 wings is provided by the family of large Asiatic timber trees,' the Diptero- 

 carpaceae (Fig. 1414). The fruits are generally one-seeded by reduction 

 and are large, the nut being from half an inch to two and a half inches long, 

 and quite heavy. Two, three or all five of the sepals, in diflferent genera, 



may develop into long narrow wings, upwards of 

 six inches long in some species, and set obliquely 

 so that they impart rotation to the falling fruit 

 and delay its descent. Some of these trees reach 

 an immense height, upwards of 180 ft., and they 

 do not fruit until they are well grown and about 

 30 years old, so that the fruits have good clear- 

 ance in falling. Yet Ridley has shown that the 

 fruits generally do not travel more than 40 yards 

 and that 100 yards is about the extreme range in 

 high winds. At this rate the species Dipterocarpus 

 grandiflorus, which ranges from Malaya to the 

 Philippines (Fig. 1414), would have taken at least 

 one and a half million years to travel the whole 

 distance and probably double this period. Lighter 

 fruits will naturally travel further than this but 

 there seems much evidence that the spreading of 

 large trees is a slow process under any conditions. 

 A persistent and inflated calyx is sometimes 

 effective in aiding distribution of herbaceous 

 plants; as in Trifoliiim fragiferiim and Anthyllis 

 vulneiaria, where the legume fruits are small and 

 are enclosed in a papery calyx hood. The fruits 

 of Rtimex are also enclosed by expanded sepals 

 which, in some species, act as wings for wind 

 carriage, e.g., Rumex acetosa, R. crispiis, but in others each sepal is provided 

 with a corky boss on the midrib, which can only be understood as a float 

 for water or sea dispersal. 



Petals which function as organs of flight are less common but can be 

 seen in many Ericaceae, especially species of Erica, where the capsules 

 dehisce inside the dry, papery corolla tube, which is detached and blown 

 away with its load of seed. Calluna behaves in similar fashion, but the 

 sepals as well as the petals take part. The withered corolla remains attached 

 to the fruits in some species of Trifolium, such as T. repens, and is detached 

 and blown away with the fruit. 



Bracts, either singly or conjoined into involucres, quite commonly form 

 wings. Three common plants in the British Flora illustrate this function 

 well. In Tilia, the Lime Tree, the bract is partially adnate to the pedicel, 

 its upper part only being free. The pedicel is detached with the bract 



Fig. 



1 4 14. — Dipterocarpus 

 graudiflonis. Fruit with 

 two large sepaline wings. 

 (After Ridley.) 



