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



5. The degree of shortening varies according to the age of 

 the stamens, the temperature, and other influences which exalt 

 or reduce their irritability, as, for instance, the integrity of the 

 flower, and the condition of the other filaments when irritation 

 is applied to one of their number. 



6. The medium degree of shortening, in thirty-one measure- 

 ments, was rather above twelve-thousandths of a Viennese line, 

 or about one-eighth of the length of the filaments. Thus, a fila- 

 ment which, when extended, is 12 millimetres in length, shortens, 

 when touched, to 10*5 mill. This estimate is within the truth; 

 for the whole amount of contraction cannot be measured. 



7. Immediately after the shortening has attained its maxi- 

 mum, elongation commences, and proceeds much more rapidly 

 at first than subsequently. The curve, consequently, is much 

 more abrupt at first, and of a larger arc afterwards. The same 

 law obtains in the case of muscle after irritation. 



8. The interval between the maximum contraction and the 

 maximum extension varies in length : the medium time is about 

 ten minutes ; but in some instances only six, and in others fif- 

 teen, minutes elapse. The irritability depends greatly on the 

 age of the flower : it is greatest when the style has not yet fully 

 extended itself beyond the yet closed anthers encircling it ; and 

 it is lost when the style has reached its maximum length and 

 the anther-cells are divergent, although the corolla do not then 

 show the least sign of withering. It therefore follows that the 

 period at which the stigma can be impregnated is subsequent to 

 the loss of irritability on the part of the stamens. 



9. By repeated irritation, a maximum contraction may be ob- 

 tained and kept up for some time. Whether the irritability of 

 the organ undergoes diminution, and may be eventually de- 

 stroyed by long repeated excitation, is not determined. The de- 

 cision of this question is theoretically of much moment ; for if 

 such decrease and loss occur, then the phenomenon of fatigue, 

 as witnessed in muscular fibre, ranks also as a property of the 

 irritable substance of plants. To solve the question, an appeal 

 may be made to other plants exhibiting irritability, such as the 

 Berberis, Mimosa, Drosera, and Diojuea; and in the two last- 

 named examples experiment has positively shown that too often 

 repeated contact paralyzes the irritability of their leaves. 



10. By prolonged irritation of the stamens, their subsequent 

 extension is found to decrease progressively, both in degree and 

 in the rapidity with which it occurs. 



11. The capacity of shortening themselves, even irrespectively 

 of the irritation of an external excitant, continues in the fila- 

 ments for a considerable time, though it gradually declines. 



12. At first sight it might be supposed that the shortening 



