390 
PHYSIOLOGY. 
BOOK II. 
than in deciduous or 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 Recueil des Prix 
de VAcademie de Bordeaux, vol. iv., that the juice of this plant 
is free from all the objections usually found to the red colour- 
ing matter used for such experiments, and that he had suc- 
ceeded 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 pecu- 
liar 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 general 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 
demonstrated by Biot, who, by means of an apparatus de- 
scribed 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 in- 
fluence 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 wea- 
ther, 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, the earth at the foot of the 
tree freezes ; and, whether at that time the roots are mecha- 
