680 MOLECULAR FORCES IN THE PLANT. 



xylein, 1ms 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 (Polyanfhes tuberosd) and 

 Snapdragon {Antirrhinum majtis) 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 Dali/ra 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. a I'etude de la Chimie moleculaire, p. 553) 

 that plants of Impatiens parvijiora, 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.] 



2 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 expeiiments, from the saline solution not rising so fast as pure water: (also Trans. Roy. 



