25 



Besides these contrivances for dissemination, such as valves, springs 

 and explosions, many others may be noticed of an hygroscopic na- 

 ture ; but amongst the various contrivances none can scarcely exceed 

 the ingenuity displayed in those parts which are destined for aerial 

 flights, and which cannot be exceeded by the skill of the aeronaut : 

 notwithstanding Nature does not inflate a hollow bag with a buoyant 

 gas, still she fabricates a machine which is capable of being wafted on 

 the lightest winds, carrying with it a parachute containing the living 

 germ to ripen its produce in some distant spot ; — such contrivances 

 being mostly manifested in the Composite, and especially among the 

 Thistles. 



There is another act performed by the aid of moisture and dryness 

 which approaches somewhat near to locomotion : this occurs in the 

 fruit of Erodium, and in the parts surrounding the fruit of Avena fa- 

 tua, commonly known as the "animated oat" where, by the twisting 

 by dryness and untwisting by moisture of the awn, change of place is 

 the result. 



Notwithstanding the parts selected here as examples are entire or- 

 gans, and where the hygroscopic action is most marked, still all parts 

 of plants and all tissues are more or less susceptible of absorbing a 

 certain amount of moisture, which causes an increase in their bulk. 

 This is seen in woody portions, as in doors, which close well enough 

 in summer, but are so enlarged by the damp of winter as to be closed 

 with difficulty, yet regain their natural size on the return of the dry 

 season. This reciprocating condition of ligneous parts has been sa- 

 tisfactorily and ingeniously shown by Mr. Edg worth, who has con- 

 trived to convert this alternate state of expansion and contraction into 

 a power acting in one direction, and thereby he has contrived to con- 

 vert a peculiar form of wooden stool into a locomotive machine.* 



Enough has been said to establish the broad principle of the hygro- 

 scopic properties of vegetable tissues in particular instances, and in 

 the next place it becomes necessary to examine if, where these cha- 

 racters are so evident and accompanied with such marked effects, 



* The stool was made of wood that absorbs moisture readily (deal cut endways from 

 the plank was used by the author), and into it fixed four legs, inclining all in one direc- 

 tion, at about 20° with the top, each terminated with a point of metal, which, from the 

 inclination of the legs, would only allow the stool to be moved in one direction. As it 

 expanded by moisture it would cause one pair of legs to recede a small space from the 

 other, and when it contracted, the legs that first receded could not be again brought 

 to the spots from which they moved, but the legs which remained fixed by expansion 

 now will be moved by contraction, and by the changes occasioned in the wood by ex- 

 pansion and contraction, caused the stool to move always in the same direction. 



