July 22, 1904.] 



SCIENCE. 



119 



WEIGHT OF DANDELION-DOWN. 



How heavy is a dandelion-down with its 

 achene as it floats away on the breeze? The 

 tiny parachute is so constructed that the 

 weight of the achene brings the spreading rays 

 at the summit of the slender rod (rostrum) 

 into the proper position for floating in the 

 air, as any one may readily see for himself 

 by ' blowing ' a seeding dandelion head in a 

 quiet place. It makes no difference what the 

 position of the parachute may be, as soon as 

 it is free in the air the weight of the achene 

 rights it at once. One who has not closely ob- 

 served dandelions will be much interested 

 in watching the quick ' righting ' of every 

 little parachute under the action of the tiny 

 achene weight. Recently it occurred to me 

 to find out how heavy dandelion-downs are, 

 and at my suggestion Mr. d'Allemand under- 

 took the delicate task of weighing them. He 

 found that there are about one hundred and 

 ninety achenes in each dandelion head, and, 

 carefully counting this number, he determined 

 their aggregate weight to be .085 gram. From 

 this it was easy to calculate the weight of a 

 single achene to be .00044 gram. It takes 

 more than two and a quarter millions of 

 dandelion-downs to weigh a kilogram, and 

 somewhat more than one million to weigh a 

 pound. In other words each parachute weighs 

 about one millionth of a pound ! 



TENDEILS OF VIRGINIA CREEPER. 



It is pretty generally knovra nowadays that 

 some Virginia creepers cling to walls by dis- 

 coid expansions of their tendril tips, while 

 others produce twining tendrils without such 

 expansions. Among gardeners there is a pretty 

 general notion that there are two quite dis- 

 tinct kinds, distinguished mainly by the pres- 

 ence or absence of disks. This distinction has 

 even been admitted into recent descriptive 

 manuals, as in Britten's 'Manual,' where the 

 disk bearing form is called Parthenocissus 

 quinquefolia, and a form with ' tendrils mostly 

 without terminal adhering disks ' is set off as 

 the variety laciniata. 



In a recent popular article Professor Pam- 

 mel gives his opinion that this difference as to 

 the formation of disks is not constant with 



any particular plant, and in a subsequent let- 

 ter cites the case of the planting of a disk- 

 bearing Virginia creeper which later formed 

 ordinary twining tendrils only. On smooth 

 surfaces the disks are not produced. This 

 agrees with the statement made by Goebel in 

 his ' Organography of Plants ' (page 268, 

 English edition) as follows: " Mohl was the 

 first to show that the adhesive disks on the 

 tendrils of certain species of Ampelopsis ap-, 

 pear in consequence of contact with a firm 

 body. We have here to do with a contact 

 stimulus. Different species of Ampelopsis 

 behave differently. Some, like A. hederacea, 

 possess ordinary tendrils which twine round 

 a support and eventually become firm, woody 

 structures, but if they do not happen to find' 

 a support they die off at an early period. 

 Ampelopsis quinquefolia, on the other hand, 

 fixes itself to walls and tree trunks by means 

 of adhesive disks on its tendrils, but these 

 can also act like ordinary tendrils. In ten- 

 drils which do not come in contact with a firm 

 body no viscid disks appear." Making allow- 

 ance for some confusion as to the identity of 

 the species, it appears that Goebel regards the 

 formation of disks as a result of a mechanical 

 stimulus. There is need of a number of care- 

 ful observations on this point in connection 

 with one of the most widely grown of all orna- 

 mental climbing plants. 



Charles E. Bessey. 

 The Univeksity of Nebraska. 



FIELD WORK FOR 190.', OF THE DIVISION 



OF GEOLOGY AND PALEONTOLOGY OF 



THE UNITED STATES GEOLOGICAL 



SURVEY. 



The field work of the division of geology 

 and paleontology. United States Geological 

 Survey, for the season of 1904 will cover in- 

 vestigations in many states. Dr. C. Willard 

 Hayes, geologist in charge of geology^ has 

 general supervision of this work. Some of 

 the most important of the numerous parties 

 in the field are here mentioned. 



General Investigations. — Besides investiga- 

 tions confined to the limits of one or two 

 states, several lines of work will be taken up 

 that will cover wide general areas. -The gla- 



