30 



and the rays collapse to their original position. Notwithstanding this 

 mechanical explanation may account for the fact in some way, yet 

 some of the rays take occasionally a direction that leads me to believe 

 that besides this force arising from the hygrometric nature of the 

 ring, they possess in their structure individually hygroscopic proper- 

 ties, and are capable, like the Geranium and others, of moving and 

 taking a definite direction by the loss of moisture. 



The above instances, though numerous others might be selected, 

 give illustrations of the action of the evaporation causing the separa- 

 tion of parts, opening of valves, and the expansion of various organs 

 of plants, but it happens in the species of Epilobium and Oenothera, 

 that moisture performs the same part. In these it will be found that 

 tissues having different contractile powers are opposed to each other 

 as before, but in the reverse method to that which has been described. 



Such are sufficient examples to prove that wherever change of po- 

 sition or form occurs by moisture or dryness, there is some peculiar 

 structure of the part; — the tissues having different hygroscopic pro- 

 perties, and these tissues so arranged as to be antagonists to each 

 other. For if there were not this antagonism, the parts being com- 

 posed only of one tissue of uniform density, there would be merely 

 shrivelling in length or thickness, and we should never find spiral or 

 curved portions in any organ occurring from loss of moisture. Whilst 

 these several parts are young, no displacement occurs, the balance of 

 this power in the tissues being preserved, but as soon as the equili- 

 brium is disturbed, then the effects before mentioned ensue. 



To account for these actions consequent on the hygroscopic pro- 

 perties the theory of endosmose has been brought forward, which is 

 ingenious, and would be satisfactory if the arrangement of parts were 

 always in accordance with that theory ; when dryness performs the 

 duty, we should expect the dense tissue to be on the reverse side of 

 the curve to which it is found. 



If we examine the section of the valve of Cerastium, it is seen that 

 the most dense tissue is on the concave surface, the same in Gerani- 

 um : yet the application of moisture straightens these organs instead 

 of increasing the curve ; consequently endosmose will not explain the 

 phenomenon in all instances. 



When in those organs which, by loss of moisture, have the conca- 

 vity of the curve on the side where the tissue is most dense, one of 

 two circumstances must happen in producing it: — 1. Either that the 

 natural condition of the dense tissue is elastic, and having a tendency 

 of its own to bend, and when not opposed by a counteracting force 



