MOVEMENT OF WATER IN PLANTS. 
681 
place in a solution of lithium citrate, and then examined the ashes of successive inter- 
nodes by the spectroscope. The solution was found to rise from 42 to 46 cm. in one 
hour. But neither method of calculation is exact or probably of much value. 
The current of water in the wood which replaces the loss occasioned in the leaves 
by transpiration is not caused by osmose, since at the time when the transpiration is 
strongest and therefore the current in the wood quickest, the cavities of the conducting 
wood-cells do not contain sap but air, or at the most are only partially filled with fluid. 
If the rising of the water took place by endosmose from cell to cell, the cells would 
necessarily possess closed cell-walls and be full of sap, the concentration of which would 
constantly increase from below upwards in the wood. But the conducting cells are at 
this time not closed, but partially or altogether (as in Coniferae) connected with one 
another by open bordered pits \ In the spring, before strong transpiration sets in, ai^d 
therefore at a time when the water in the wood is comparatively at rest, the wood- cells 
also, it is true, contain sap, flowing in quantities out of their communicating cell-cavities 
when holes are bored in the trunks (as in the Birch, Maple, &.C.). But this sap does 
not, as is proved by analysis ^, show a concentration increasing from below upwards. 
The fact^ also that water rises in cut leafy stems placed with their upper end in water 
though planted and rooted, and flows therefore in a direction opposite to the ordinary 
one in the stem, shows that endosmose depending on a definite distribution of the 
concentration of the sap cannot be the cause of the current of water. Since vessels 
and wood-cells communicating with one another through their open pores form narrow 
cavities which sometimes become wider as they proceed, sometimes narrower, the woody 
substance may be represented by a bundle of narrow glass tubes alternately bulging and 
contracting, in which the water which fills them rises by capillary attraction. But how 
little efftcacious a contrivance of this kind would be is seen at once from the width of 
the capillary tubes, which is much too great to raise water to a height of 100 feet or 
more. It must also be pointed out that in the summer, when the current of water 
is strongest, it is principally air and not fluid that is conveyed through the cavities of 
the cells. 
Since it is evident from what has been said that the movement of the water takes 
place in the woody substance and not in the cell-cavities filled with water, there remain 
only two hypotheses; 'viz. (i) that the movement takes place in the water contained 
Irish Acad. vol. XXV, 1874). Sachs has found that salts of lithium do travel along cell-walls as fast 
as the water in which they are dissolved. By supplying the roots of plants with a solution of a salt 
of lithium, he has obtained the following rates at which it travelled in the root and stem : — 
Plants with roots in water. Rate per hour. 
Salix fragilis . . . . 85*0 cm. 
Zea Mais ..... 36'0 „ 
Plants with roots in earth. 
Nicotiana Tabaciim . . . 1 18 0 
Albizzia lophantha . . . 154"0 
Mnsa Sapientum .... 99*7 „ 
Heliaiithus aiiniius . . . . 630,, 
Vitis vinifera . . . . 98*0 „ 
In all these cases the plants were under such conditions as to promote transpiration to the utmost. 
(Sachs, Beitr. z. Kennt, d. aufsteigenden Saftstroms in transpirirenden Pflanzen, Arb. d. bot. Inst, in 
Würzburg, II. i, 1878.] 
^ [Sachs has found that in Abies pectinata the bordered pits of the spring-wood are closed 
(Porosität des Holzes).] 
^ The older statements of Unger are referred to in my ' Experimental-Physiologie ; ' others will 
be found in Schröder, Jahrb. für wiss. Bot. vol. VII. p. 266 et seq. 
^ The conduction is however by no means so considerable in the reversed as in the ordinary 
direction, as Baranetzky found in the laboratory at Würzburg ; but this may be connected with other 
peculiarities of the organisation. 
