November 17, 1911] 



SCIENCE 



683 



DISCUSSION AND COHRESPONDENCM 

 A NEW TOY MOTOR 



I MADE of wood a nacelle about two inches 

 long, pointed at one end and open at the 

 other, shaped like a skiff without a stem- 

 board. It was rendered water-repellent by a 

 slight coating of paraffin. A slice of soap was 

 fitted into the stern and the boat thus com- 

 pleted was placed on still water in a bath tub. 

 As was anticipated, the craft began to move 

 off as soon as the water came in contact with 

 the soap. After gathering way it reached a 

 velocity of a couple of inches per second. 

 Sometimes the course was nearly straight, 

 sometimes erratic, as might have been ex- 

 pected in the absence of steering apparatus. 



The power is derived from the potential 

 energy of the surface water-film set free by the 

 diminution of surface tension, this reduction 

 being due to solution of the soap. 



If the whole immersed surface of the boat 

 is allowed to become soapy, converse condi- 

 tions set in. The boat is then approximately 

 in stable equilibrium in the center of an area 

 of low surface tension and, if displaced by a 

 half an inch or so, may return to its place 

 almost as if anchored. 



It seems a •priori improbable that the means 

 of locomotion illustrated by this little motor- 

 boat has not been utilized in nature. If, for 

 example, the ripe seeds of a plant growing in 

 shallow, still water were boat-shaped and pro- 

 vided with a store of soluble material at the 

 blunt ends, they might attain a much wider 

 dissemination or more varied environment 

 than that open to similar seeds not fitted to 

 utilize the potential energy of surface tension. 



I am not aware that such seeds have been 

 described, but my acquaintance with botan- 

 ical literature is of the slightest. If the facts 

 are already known this note may assist to dif- 

 fuse a knowledge of them. 



George F. Becker 



Washington, D. C, 

 October 27, 1911 



A COMMON error CONCERNING CECIDIA 



It is well known that many errors which are 

 recognized by scientific workers are repeated 



in various publications, including text-books, 

 until they threaten to become as thoroughly 

 engrafted into our literature as the George 

 Washington hatchet and cherry-tree story, al- 

 though not nearly so useful. Among these 

 errors is the prevailing opinion that vegetable 

 galls which are due to insects are the result of 

 an irritating fluid secreted by the female 

 parent insect at the time of ovipositing. 

 Many of our scientists cling to this ancient 

 theory as tenaciously as the young American 

 clings to the wonderful hatchet story. 



The latest outbreak is in the recent edition 

 of the Encyclopaedia Britannica, in which, 

 under the heading " Galls," it is said that 

 " The exciting cause of the hypertrophy, in 

 the case of typical galls, appear to be a minute 

 quantity of some irritating fluid or virus, 

 secreted by the female insect, and deposited 

 with her egg in the puncture made by her 

 ovipositor in the cortical or foliaceous parts of 

 plants. This virus causes the rapid enlarge- 

 ment and subdivision of the cells affected by 

 it, so as to form the tissues of the gall. Oval 

 or larval irritation also, without doubt, play 

 an important part in the formation of many 

 galls." 



In consideration of this prevailing idea it 

 may be worth while to review our knowledge 

 on this point. This theory was first advanced 

 by Malpighi in his " De Gallis " (1686), who 

 believed that the female parent secreted a 

 poison when she deposited the egg and that 

 this caused a fermentation of the plant acid 

 which stimulated the plant cells and thus 

 caused the gall. This theory was repeated 

 almost without question until the latter part 

 of the last century; Reaumur accepted it but 

 thought that the egg might have some thermal 

 effect and that the character of the wound 

 might also be a factor; Dr. Derham said it 

 might be " partly due to the act of the plant, 

 and partly to some virulency in the juice or 

 egg, or both, deposited in the vegetable by the 

 parent animal; and just as this virulency is 

 various according to the difference of its ani- 

 mal, so is the form and texture of the gall 

 excited thereby " ; Darwin expressed the opin- 

 ion that galls were caused " by a minute atom 



