686 
MOLECULAR FORCES IN THE PLANT. 
form shown in Fig. 467, R is fixed to the stump, and to it is attached a narrow tube 
instead of the manometer ; the free end of this tube is bent downwards into a graduated 
burette. If the tubes are from the first filled with water, as much runs into the 
burette as flows out from the cut section, and the pressure therefore remains constant. 
From this experiment it has been ascertained that the flow of water varies from day to 
day, from one time of the day to another, and even from hour to hour ; but the causes 
of these variations in the outflow, which must depend on the activity of the roots, are 
not yet knowm ; it would even seem as if a periodicity were established independently of 
the temperature and of the moisture of the ground 1. 
The measurement of the highest pressure at which the outflow can still take place at 
the cut surface can be eff'ected by the apparatus figured in Fig. 467, where it is expressed 
by the difi'erence of level of the mercury in the two arms of the tube, or by q — q'. 
This will however only afford a measurement of the pressure which the outflowing \^ ater 
may be able to overcome at the cut surface ; but it has obviously had also to overcome 
other resistances of unknown magnitude in the interior of the root-stock. With respect 
to this point I was interested in ascertaining how great is the diff"erence in the outflov/ if 
one of two equal root-stocks has no pressure to overcome at the cut surface, the other 
a considerable but constant pressure. If, in Fig. 469, a indicates the cut stem of a 
Sunflower or similar plant grown in a pot, c, d, e the tube which is attached to it by the 
india-rubber tube b, and / a glass tube bent downwards, which (not as in the figure) 
reaches beyond the rim of the pot and terminates in a burette, while the opening of / 
lies exactly on the level of the cut surface of the stem; then, when the tube c, d, e,f 
has been filled with water, we have an apparatus for observing the outflow when the 
pressure at the cut surface is at zero. A second root-stock from a plant of exactly 
the same age and vigour and grown in a pot of the same size is provided with the 
apparatus figured in Fig. 469, where the tube f through which the outflow takes 
place reaches the vessel h through the cork g. This vessel contains water in its upper, 
mercury in its lower part. A tube k rises from the cork i to a certain height and is bent 
round at the free end 0 where it dips into a graduated tube. If the apparatus is so 
contrived that, for example, the opening for the outflow 0 stands about 1 5 cm. above the 
level n, then the column of mercury oji exercises a pressure of 15 cms. on the water 
and through it on the cut surface at b. When the water begins to flow out from the cut 
surface at b, the quantity of water in h will be increased, and an equal volume of mer- 
cury will flow out at 0. The mercury collects in the burette, and its level enables 
the quantity of water which has flowed from the cut surface to be read off" from hour 
to hour, and to be compared in the other apparatus where there is no pressure. After 
a long period of observation, the level n falls sensibly and the pressure on augments a 
little. But it is easy to bring it again to the original amount if a fresh quantity of mercury 
is poured in every twelve hours. 
^ Very detailed observations on this point were made by Baranetzkyin the Würzburg laboratory, 
in the summer of 1872. For this purpose he availed himself of the autographic auxanometer described 
hereafter. The sap flowed from the root-stock into one limb of a long narrow U-shaped tube, in the other 
limb of which was a float bearing an index which marked the changes of level upon the smoked paper 
of the rotating cylinder. This method gives very accurate results in so far as the quantity of the sap 
is concerned. Considerable variations of temperature (10° in 24 hours) affect the flow in such a way 
that every rise increases, every fall diminishes it. If the variations of temperature are small, a daily 
periodicity which is independent of such variations can be detected, a maximum and a minimum being 
attained daily. According to Baranetzky, the time of their occurrence depends upon the periodical 
exposure of the plant to light during the time preceding the experiment. I am unable to assure 
myself of the correctness of this conclusion from Baranetzky's experiments. I do not deny the 
possibility of it, but I reserve my opinion until further experiments have been made. (See 
Baranetzky, Bot. Zeitg. 1873, No. 5, and Die Periodicität des Blutens, Abhandl. d. naturfor. Ges. zu 
Halle, XIII, 1873.) 
