STRUCTURE OF FRUIT. 181 



long, as ill Gossypium, where they form the substance so 

 celebrated and useful, called Cotton ; sometimes they are 

 found scattered over the whole surfaces of the epidermis, 

 as in Ochroma ; sometimes only in certain places, as in 

 several varieties of the Cotton Plant : sometimes in a 

 tuft at the extremity of the seed, as in several Apo- 

 eyneae. These tufts, which have been called Crests, 

 {comae) so resemble pappi, that they have frequently 

 been confounded with these organs ; but they present 

 this essential difference — that the pappus, which is a 

 degeneration of the limb of the calyx, is on the outside 

 of the pericarp, and the crest, which is an extension of the 

 epidermis, is within the cells of the fruit, and upon the 

 seed itself. Notwithstanding this important anatomical 

 difference, their nature and properties have a great 

 analogy. These two kinds of tufts are formed of mem- 

 branous and very hygroscopic hairs, endowed with the 

 faculty of approximating when moist, and diverging 

 when dry ; whence it results, that as long as the ma- 

 turation has not been completed, these hairs, being 

 moist, remain close together ; but becoming dry at ma- 

 turity, they diverge, and thus tend to facilitate the 

 bodies to which they are attached in coming out of their 

 envelopes. The pappus draws the achenium out of the 

 involucrum ; the crest carries the seed out of the peri- 

 carp ; both of them, being expanded, permit the least 

 wind to bear to a distance these little bodies, to which 

 they perform the office of wings, or rather of parachutes. 

 I shall now revert to the modifications of the epidermis. 

 It frequently happens that this membrane is ex- 

 panded around the seed, and, instead of bearing hairs, 

 is dilated into a wing frequently well developed and very 

 delicate. It is thus that in several Apocyneae, Malvaceae, 

 &c. the seed is terminated, surrounded or enclosed by a 

 membranous wing, which, like the crest, contributes to 



