2i8 THE MOVEMENTS OF WATER 



fortunately, the inaccurate use of these important methods has often led to erroneous 

 conclusions. Above all it must be borne in mind that the dye, like water, diffuses 

 in all possible directions from the conducting elements, so that ultimately those 

 cell-walls and cells will be most deeply coloured which absorb and accumulate the 

 colouring material, even though such parts may not function at all in the conduction 

 of water '. Before any conclusions can be made, the dependence of the distribution 

 of the colouring material upon the water-current which carries it must be established, 

 and the tissue-elements in which the dye first appears must in all cases be deter- 

 mined. For purposes of demonstration, white flowers of Crocus, Iris, Lilium, 

 Primula stnensis, &c., may be advantageously employed. When the cut stalks of 

 the flowers or inflorescences are placed in a deep blue solution of indigo-carmine, 

 the first appearance of the coloured fluid in the fibrovascular strands may be 

 observed and its gradual progress followed as transpiration continues. If twigs 

 are used, the progress of the dye can be traced by making a series of transverse 

 sections. For researches with trees, the oak or lime will serve, and the use of 

 eosin or indigo-carmine is to be recommended, whereas methyl-blue, fuchsin, and 

 a few other dyes are less suitable, for reasons given later z . 



That the dye is actually carried along with the water-current is shown by 

 the fact that in strongly transpiring plants it usually travels 0-5 to 3 metres 

 forward in an hour, whereas without any transpiration it travels barely i cm. 

 in the same time. In every case the plant must be previously saturated with 

 water, and cut stems or stalks must be kept at least half to one hour in water 

 before use, since otherwise the negative pressure in the opened vessels would cause 

 the coloured solutions to be sucked in quite independently of the transpiration 

 current 3 . Entire plants are not so well adapted for such experiments, for the 

 dye passes with much greater difficulty through the living epidermal and cortical 

 cells of the root than water does. Thus water can pass directly through the cells, 

 whereas bodies which cannot diosmose through the protoplasts, such as indigo- 

 carmine, aniline-blue, or nigrosin, travel only through the cell-walls. Hence, owing 

 to the relatively small sectional area of these, such substances are transferred with 

 comparative slowness 4 , and even dyes which penetrate the protoplast travel more 

 slowly than water does. Moreover, the dyes which can be absorbed, eosin included, 

 exercise in general a poisonous influence even when very dilute, and this accelerates 

 the transference of the dye, whereas other causes (passive secretion, &c.) tend to 

 retard its passage. Coloured solutions first appear in the tracheae and tracheides 

 of the alburnum, whence diffusion to the surrounding tissue-elements soon occurs. 

 This result is best obtained with indigo-carmine, which is not absorbed or secreted 

 to any noticeable extent, and the use of a deeply-coloured saturated solution makes 



1 For examples see Sachs, Arb. d. Bot. Inst. in Witrzburg, 1875, Bd. n, p. 150. Cf. also 

 Schwendener. Sitzungsb. d. Berl. Akad., 1892, Bd. xi.iv, p. 925. 



2 See Strasburger, I.e., pp. 551, 566. 



3 Cf. Sect. 32 ; also Sachs, Arb. d. Bot. Inst. in 'Wtirzburg, 1878, Bd. n, p. 157; Strasburger. 

 1. c., 1891, p. 589. 



4 Cf. on this and the following, Sects. 15, 16, &c., where Pfeffer's researches with dyes are given 

 (Unters. a. d. Bot. Inst. z. Tubingen, 1886, Bd. n, p. 268). Wieler (Jahrb. f. wiss. Bot., 1888, Bd. XIX, 

 p. 119} has recently employed entire plants. 



