178 



the most striking instances from four to six inches in three feet. 

 According to Leavitt, the longest horizontal distance of flight 

 measured by him was 25 inches, though he adds : " The range 

 of the trajectory may be three or four feet, at die greatest." (Ital- 

 ics mine.) 



Now, assuming that the normal range of flight for these gemmae 

 is three feet, and that they fall only four inches in that distance, 

 which gives us very favorable data for an estimate, the gemmae shot 

 out from the ' up side ' of a plant growing on a slope with one 

 foot rise in three, would reach a distance of only about sixteen 

 inches, measured on the slope, even if we assume a horizontal 

 flight. If the plant be tilted so that its axis is at right angles to 

 the slope, the force which would carry the gemma to a distance 

 of three feet, with a vertical rise of one foot, would carry the ob- 

 ject approximately six feet, horizontal distance, a conclusion quite 

 at variance with the observed facts. 



It would seem strange, too, that the gemmae which Leavitt 

 saw were to be found only in one direction from the nearest adult 

 plants as it is well enough known that they grow out in various 

 positions on the stems.* An examination of these plants would 

 have shown the basal parts of the gemmiparous branches, and 

 Leavitt's conclusion may thus have been tested. To be sure, 

 there is also the possibility that the conditions in other direc- 

 tions were not favorable for germination ; but in the absence of 

 definite evidence to the contrary we must assume that they were. 

 There is, therefore, a considerable degree of probability that the 

 plantlets referred to had some other origin. 



Finally, the supposition that in order to initiate the propulsion 

 of the gemma it must be struck by some moving object, may 

 be beyond the facts. We may as justifiably assume that the 

 mechanism is complete in itself, and that the tensions set up in 

 the clasping leaves are sufficient to cause the phenomenon. Our 

 observations touching this point are incomplete, and it is there- 

 fore better to hold the judgment in abeyance. 



Movements of Leaflets in Onoclea sensibilis. — The statement 



*Goebel has pointed out in the Organography (p. 651) that the gemmae do not 

 grow on the side of the stem turned toward the sister stem of a fork but this, of 

 course, does not affect our statement. 



