324 MOTION OF FLUIDS, PROOFS OF. [BOOK n. 



herbaceous plants, and the epidermis thicker and harder, 

 perspire much less than other plants. 



M. Biot has succeeded in injecting the red colouring matter 

 of Phytolacca decandra into the flowers of white hyacinths. 

 He learned from a paper by De la Baisse, in the Recueil des 

 Prix de I'Academie de Bordeaux, vol. iv., that the juice of 

 this plant is free from all the objections usually found to the 

 red colouring matter used for such experiments, and that he 

 had succeeded in injecting it into all sorts of white flowers, 

 and even green leaves. Biot found, however, that although 

 he did in many cases succeed, yet the practice was attended 

 with peculiar difficulties. Many plants refused the injection 

 altogether, others took it up with rapidity. A few minutes 

 sufficed to vein with a multitude of red lines all the petals 

 of a white monthly rose, while a white musk rose was not 

 affected. He even found that the flowers of the same species 

 resisted the entrance of the colouring matter in an unequal 

 degree. 



That a motion of fluids really exists in plants is, therefore, 

 undoubted. It is most rapid in the spring and early summer, 

 and most languid in winter ; but never actually suspended, 

 unless under the influence of frost. This has been demon- 

 strated by Biot, who, by means of an apparatus described in 

 the Institut Newspaper, succeeded in measuring the power of 

 motion in the sap of plants, in witnessing the phenomena 

 which regulated it, and in determining the causes that 

 brought them about. 



" Atmospherical circumstances/' he says, " and especially 

 the absence or presence of solar light, exercise a marked 

 influence upon these phenomena ; but it is exceedingly 

 difficult to ascertain their exact nature. Nevertheless, 

 among them is one, the effects of which are so constant and 

 undoubted, that they appear susceptible of being defined. 

 This consists in the sudden appearance of frost immediately 

 succeeding mild weather, and lasting for some time. Mild 

 weather either favours or brings about the ascent of the sap ; 

 but, if a sudden frost supervenes, it seizes upon the part of 

 the trunk swollen with fluid, and forces the latter to fall 

 back again : should the frost continue and increase in severity, 



