68o 
MOLECULAR FORCES IN THE PLANT. 
xylem, has already been stated. It can be demonstrated in a more conspicuous manner 
by placing a cut stem or branch with its cut surface in a coloured solution ^ while the 
leaves are transpiring. If the stem or branch is cut through at various heights after 
a few hours, or according to circumstances after a longer period, the colouring of the 
wood will show how high the solution has been sucked up in it, and will be seen only in 
the woody bundles and not in the cortex or pith. If branches with pure white flowers 
are employed in this experiment, such as a white-flowered Iris or Deutzia, and if they 
are placed in a dark aqueous solution of aniline, the white petals are found, after from 
ten to fifteen hours, to be permeated by dark blue veins corresponding to the fine woody 
bundles of the venation. This beautiful appearance however soon vanishes, the poisonous 
colouring material subsequently killing the adjoining layers of parenchyma, and colouring 
the spaces between the veins blue by diffusion, and the corolla consequently becomes 
flaccid ^. 
The difference in the amount of transpiration under different external conditions 
must also correspond to a difference in the rapidity of the current of the water in the 
wood. In rainy weather, when there is no evaporation or but very little from the 
leaves, the movement of the water in the stem will be very slow ; but when the 
transpiration increases with sunshine and wind, the current of water in the woody 
bundles is also accelerated. Under the hypothesis that the water moves only in the 
woody substance of the walls of the wood-cells themselves and not in their cavities, I 
have calculated the rapidity of the ascending current of water in a branch of the Silver 
Poplar in which there was strong transpiration, and obtained a rate of 23 cm. per hour. 
M^Nab placed branches of Prunus Laurocerasus^ from which transpiration was taking 
^ I must take this opportunity of making the remark that I still entertain, and in a high degree, 
the doubt previously expressed, whether it is not a purely pathological phenomenon that is produced 
in this manner. 
^ [This is a method of experimentation which has been practised by numerous observers since 
the commencement of the last century, when it was apparently first tried by Magnol. Sarrabat 
(otherwise Delabaisse) coloured the veins of the flowers of the tuberose {Polyanthes tiiberosa) and 
Snapdragon {Antirrhifium majus) by watering the plants with the juice of the berries of Phytolacca. 
(Dissert, sur la circul. de la Seve, Bordeaux, 1733.) 
Van Tieghem (in the French edition of this work, p. 791) quotes Reichel as having plunged the 
roots of a flowering plant of Datura Stramonium into a decoction of the wood of Fernambouc ; the 
liquid followed the course of the vessels, and after eight days veined the corolla with red, and made 
its appearance also in the stamens, the walls of the fruit, and even in the style. (De vasis plantarum 
spiralibus, Leipzig, 1748.) For other old authorities see De Candolle, Phys. Veg. i. 82. 
De Saussure found that the stem of a bean became coloured by a decoction of Brazil-wood ; and 
this was one of the facts upon which he based the conclusion that organic matters were capable of 
being taken up by the roots of plants (Ann. des Chem, u. Phys. xlii. p. 275). Biot noticed that the 
red colouring matter of Phytolacca was absorbed by white hyacinths when poured upon the soil in 
which they were grown ; after two or three days, however, the red colour disappeared from the 
flowers, (Comptes Rendus, 1837, i. 12.) linger also made the same experiment (Botanical Letters, 
p. 38). Hallier immersed the ends of cuttings of plants in solution of indigo or black cherry juice. 
(Phytopathologie, 1868, p. 67). Persoz states (Introd. ä I'etude de la Chimie moleculaire, p. 553) 
that plants of Impatiens parviflora, the roots of which are immersed in a solution of sulphindigotic 
acid, absorb that fluid in a reduced or colourless state due to the action of the roots upon it ; in the 
petals it again undergoes oxidation and becomes blue. The experiments of Herbert Spencer (Prin- 
ciples of Biology, i. p. 538) may also be referred to.] 
^ M^Nab, Transactions of the Botanical Society of Edinburgh, 1871. [Dr. Pfitzer has suggested 
that the result may be arrived at by the much simpler mode of allowing the plant grown in a pot to 
become so flaccid from want of water that the leaves droop perceptibly, and then, after supplying the 
root with water, to observe the length of time that elapses before the leaves at various heights from 
the ground recover their normal position. Pfitzer found by this means a much more rapid rate of 
ascent indicated than that stated by M^Nab ; and believes that there is a serious source of error 
in M^Nab's experiments, from the saline solution not rising so fast as pure water: (also Trans. Roy. 
