DISPERSAL BY WIND. 



855 



When quite dry, these envelopes are extremely thin and delicate, and sometimes 

 their weight is still further reduced by a portion of the tissue being torn during 

 desiccation, in which case the whole assumes a sieve-like or latticed appearance. 

 The small fruit within the envelope defines the position of tlie centre of gravity, 

 and consequently determines also the attitude of the structure as a whole that best 

 adapts it to dispersion by the wind. In several Papilionacere, as in CaUipcUis 

 cucidlata and the yellow -flowered species of Clover (e.g. Trifoliuvi agrarium 

 and T. badiuTn; see figs. 469 *• -■ ^' *• ^), the dried petals of the corolla are fashioned 

 into an envelope which incloses the small 1-seeded legume, and in several species 

 of Lady's Fingers (e.g. Anthyllis tetrajAylla and A. Vulneraria; see figs. 4G4* 



Fig. 469.— Dispersion of fnuts and seeds by the wind. 



Trifolium badium. — i Inflorescence. - Same with fruit ripened. 8 Flower. 4 Fruit enveloped in the dried petals. 

 * Longitudinal section through the fruit in its envelope of petals.— Vertiiordia oculata. * Fruit. ' Longitudinal section 

 through the fruit. ' Five " feathers " from the fruit, s, *, ', and » magniSed. 



and 464^), and some species of Clover of the tribe Vesicastrum (e.g. Trifolium 

 fragiferum and T. tomentosum; see figs. 464^ and 464*), the inflated calyx plays 

 the same part. In many Labiates also (e.g. Calaminta, Salvia, Thymus), the calyx 

 is converted into a dry, saccate envelope, wliieh is severed from its stalk by any 

 external stimulus, and then serves as a means of dispersing the ripe nutlets con- 

 tained in it. In the Hop-hornbeam (Ostrya, see figs. 464 * and 464 '), the small nut 

 is enveloped in the sac-like bract; and in many Grasses, as, for instance, Brlza 

 mxixima and Melica altissiina (see figs. 4G8 * and 468 ''), the dry glumes constitute 

 a covering to the small fruit which adapts it to dispersion by the wind. 



One of the commonest devices for keeping fruits and seeds suspended in the air 

 is of the nature of a parachute. This form of mechanism occurs in the shape either 

 of tufts of hairs or of membranous boi-ders. In Willow-herbs (Epilobium; see 

 fig. 472*), Asclepiadaceae (e.g. Cynanchum, see fig. 471'), and several BromeliacesB 



