Prof. F. Cohn on the Contractile Tissue of Plants. 189 



To witness the phenomena of motion and irritability in the 

 stamens^ it is only necessary to isolate a floret^ as of Centaurea, 

 and to cut away one-half of the corolla in such a manner as to 

 expose the'stamens, from their point of attachment, their whole 

 length. After a period of rest of a few minutes, the filaments, 

 which have hitherto been straight and in close apposition with 

 the central style, are seen to curve themselves outwards, leanng, 

 however, their terminal anthers still closely applied to the upper 

 part of the style. This bending proceeds imtil it reaches its 

 maximum, when each filament stands out in a half-circle from 

 the style. On now touching a filament with a needle, they all, 

 so to speak, collapse and resume their vertical direction and 

 close apposition with the style. Bearing in mind the fact that 

 the filaments are fixed at their two extremities (at the upper by 

 the anthers, which are immoveable, and at the lower by their 

 insertion into the receptacle of the floret), it becomes evident 

 that, to produce the remarkable curvature they exhibit, they 

 must undergo considerable elongation. Indeed, the degree of 

 curvature does not represent the whole amount of elongation ; 

 for the filaments necessarily affect the length of the anthers by 

 the tension exercised upon them. The extent to which they pull 

 upon the anthers may be demonstrated by cutting across one or 

 more of them, when the lower half becomes drawn apart from 

 the upper, and thrust upwards above the line of section. The 

 maximum of this movement of the cut filament is stated to be 

 half a millimetre. 



The movements of the filaments in two species of Centaurea 

 (viz. C. macrocephala and C. americana) were carefully measured 

 by means of the micrometer, due regard being given to the tem- 

 perature, time of day, and other conditions likely to influence 

 the phenomenon. These measurements are given in detail; but 

 it is unnecessary to copy them here, and we shall content our- 

 selves by stating the general results of Cohn's inquiries. 



1 . The touching of a filament of Centaurea is at once followed 

 by shortening, which, in its extent, bears a direct relation to the 

 intensity of the irritation produced. 



2. The irritated filament undergoes shortening in its entire 

 length. All other parts of the flower seem incapable of a similar 

 process. 



3. The shortening commences from the moment of contact, 

 and proceeds rapidly (though not so much so as to appear in- 

 stantaneous) until it attains its maximum. A sudden act of 

 irritation, as with a needle, induces the most complete contraction. 



4. Hence it also follows that the impulse to shortening is 

 transmitted from the point irritated to both ends of the filament, 

 or from one end to the other. 



