November 3, 1905.] 



SCIENCE. 



569 



tacle-down are of such a nature that water 

 will not adhere to them. This is because the 

 surface has a stronger attraction for air than 

 for water, owing to the fact that the surface 

 is coated with a substance of an oily nature. 

 Alcohol removes this substance. 



Fig. 1. f, the fruit (achene) ; k, collar round 

 the summit of the achene, inside the edge of which 

 the pappus hairs are supported, at the base for a 

 distance of about .25 . mm., forming a ring; li, 

 the hair; M, the hairlet; c, the base of the 

 withered corolla; s, remains of the style broken 

 away. (The style and corolla usually persist 

 until the achene breaks from the pappus.) The 

 diagram to the right is an imaginary vertical 

 section. The drawing to the left is a view of an 

 achene after the down has been detached. The 

 hairs when dry are at an angle of about 45° to 

 the vertical. The hairlets are at an angle of 

 about 45° to the axis of the hair. Hence the 

 hairlets make ari angle of about 90° to each other, 

 so that at whatever angle they be placed, the re- 

 sultant effect can not be less than that of 45°. It 

 will be noticed that the collar h, being slightly 

 smaller at the extreme edge, clamps round the 

 base of the ring of hairs. This is the more effect- 

 ive because the ring of pappus is slightly larger 

 at its lower edge, making the collar act like a 

 . wedge. This is indicated in the diagram to the 

 right. 



From the nature and the position of the 

 receptacle-down, it may once have served the 

 function of moving the whole head by the aid 

 of the wind; so that the suggestion is here 

 offered that the achene-down is a later devel- 

 opment from the point of view of evolution. 



When the down, with its attached achene, 

 is exposed to dry air for a few days, the con- 

 tracting end of the achene causes it to rupture 

 from the collar of the calyx-down, and thus a 

 separation takes place eventually. This, of 

 course, is an advantage, because the seed may 

 thus reach a suitable place in the soil for 

 germination. If the pappus remained at- 

 tached, the chances for germination would be 

 greatly diminished, because the down would 

 then be but a hindrance by holding the achene 

 above the soil. In many cases, therefore, the 

 thistle-down seen floating in the air has no 

 achene attached; and this separation would 

 likely take place while floating in the air, 

 because of the favorable conditions there for 

 drying out. 



A minute examination was made of several 

 heads of thistle-down with a view to ascertain 

 to what extent the down was adapted to the 

 air conditions. In the heads there are from 

 95 to 120 achenes; or an average of about 

 108 per head; and there is an average of 80 

 hairs to one achene-cluster, and 110 hairlets 

 on each hair. These hairlets are .005 mm. in 

 diameter and 2 mm. long (approximately). 

 The surface area of each hairlet would be 

 TT X -005 X 2 ; and of all the hairlets in each 

 head : tt X -^^^ X 2 X 80 X 108 sq. mm. The 

 hairs being each .06 mm. in diameter, and 23 

 mm. long, the surface area for the whole head 

 would be: 



TT X .06 X 23 X 80 X 108. 



The cells composing the pappus are filled 

 with air when the achenes are mature, and 

 this adds to the buoyancy, but the inside sur- 

 face is not here computed because it has 

 nothing to do directly with the surface at- 

 traction of the pappus for the air of the 

 atmosphere. 



The total external surface area, therefore, 

 would be: 



