424 PHILOSOPHICAL TRANSACTIONS. [anNO 1788, 



been found combined in one and the same part. Even Sertulariae are not an ex- 

 ception to this observation. The greater part of their substance indeed resembles 

 that of plants in being indefinitely extended, and in wanting irritability and 

 spontaneous motion. But their animated flowers or polypes, in which the 

 essence of their being resides, are endued with both these properties in a high 

 degree. 



I know it is the opinion of some philosophers, that a certain degree of irrita- 

 bility must pervade every part of vegetables, as the propulsion of their fluids 

 cannot well be conceived to be accomplished by any other means. In a conver- 

 sation on this subject with the celebrated M. Bonnet, of Geneva, he informed 

 me that he is strongly of this opinion ; and that he should not despair, by throw- 

 ing acid or other stimulating injections into the vessels of some plants, of seeing 

 with a microscope at once the propulsion of the sap, and the contractions by 

 which it is performed. He urged me, with that amiable enthusiasm for which 

 he is remarkable, to pursue the inquiry. Whether I do so or not, I think the 

 idea too interesting to be kept to myself, and should be glad to see it realized 

 by any one who has time and abilities for such investigations, who has accuracy 

 and coolness in making his experiments, as well as fidelity and impartiality in re- 

 cording them. 



I cannot conclude this paper without taking notice of another very curious 

 property which vegetables seem to possess in common with animals, though cer- 

 tainly in a very inferior degree: I mean that property, to use the words of Mr. 

 Hunter, who has studied this principle to a vast extent in the animal economy, 

 by which their constitution is capable only of a certain degree of action consist- 

 ently with health; when that degree is exceeded, disease or death is the conse- 

 quence. It is only by the help of this principle that I can explain why many 

 plants resist a great degree of cold for several winters before flowering; but after 

 that critical event they perish at the first approach of cold, and can by no art be 

 preserved so as to survive the winter. But a more curious instance is that men- 

 tioned by Linnaeus, without an explanation, in his Dissertation on the Sexes of 

 Plants, of the long duration of the pistilla in the female hemp, while unexposed 

 to the male pollen ; whereas those to which the pollen had access immediately 

 faded and withered away. In this case I cannot help thinking, that in those 

 pistilla on which the pollen had acted, and which consequently had performed 

 the function for which they were designed, the vital principle was much sooner 

 exhausted than in those which had known no such stimulus. It is perhaps for 

 the same reason that double flowers, in which, the organs of generation being 

 obliterated, no impregnation can take place, last much longer in perfection than 

 single ones of the same species, as is notoriously the case with poppies, anemo- 

 nies, &c. In single poppies the corolla falls ofi^ in a few hours; but in double 



