FLUCTUATIONS IN THE DISCHARGE OF WATER 273 



place in such a manner tliat at a certain hour daily — in the present case between 

 8 and 1 1 o'clock — a maximum of the discharge occurs ; from thonce onwards 

 the quantities hourly discharged decrease into the night, to increase again towards 

 morning. It appears, however, that even with the same species of plant the hour of 

 the maximum may be considerably retarded, since another observer, who likewise 

 investigated Helianthiis animus, but grown in a flower-pot, gives the time of maximum 

 discharge as between 12 and 2 o'clock, and in general the statements of various 

 observers differ considerably on this point. Even in the same specimen, the hour 

 of the maximum discharge may alter on successive days, as our table in part shows ; 

 still for myself I always hold it to be important that a series of observations made 

 in i860, under peculiarly favourable conditions, on a plant of the common Potato S 

 likewise rooted in the open land, gave results which agree with our table in all 

 essential points, especially in that the maximum discharge always occurred in the 

 forenoon. The most remarkable fact in this periodicity lies, however, in that 

 it is independent of small fluctuations of temperature, and even of considerable 

 fluctuations of the moisture of the soil, to such an extent that in spite of these 

 it is still perceptible. Thus our table shows, on August 26, a maximum dis- 

 charge of fourteen centimetres between 8 and 9 o'clock in the morning, the tempera- 

 ture being only 13° R.; while in the afternoon, between 12 and 4 o'clock, at 19° R., 

 only 2-5 cubic centimetres were excreted, and similar relations occur repeatedly in 

 the table. In like manner the quantity of water contained in the earth is not 

 strictly proportional, since even when the soil contains less water a larger outflow 

 may take place ; however, numerous experiments in this direction teach that a drying 

 up of the soil to any great extent, as well as cooling of the roots, influences the 

 quantity of outflow. It has been attempted to represent this periodicity in the 

 activity of a root-stock as a consequence of the preceding daily periodicity of 

 illumination, so long as the plant was still intact — an assumption which is hardly 

 supported by the existing series of observations, and from general grounds possesses 

 little probability in itself. Meanwhile the cause of the daily periods is simply 

 unknown. 



The inspection of our table yields the further result that the quantity of 

 discharge at the time of the maxima is greater during tlie first two days than in 

 those which follow ; although on the 30th of August, that is on the fifth day, it 

 rises again nearly up to the original height. On the other hand, the minimum hourly 

 discharges in the later days are rather greater than at first. In other words, as 

 the experiment continues, the diff"erence between the daily maximum and minimum 

 diminishes; as is also to be gathered from my old observations of i860, on the 



^ The older literature on the weeping of the root-stock, as well as the facts from which our 

 more recent view as to the mechanism of this process has been gradually developed, are found in the 

 following works. Hofmeister—' Über Spannung, Ansfliissmcngc und Ausflussgcschwindighcit von 

 Säften lebender Pflanzen ' (Flora, 1S62, pp. 97, &c.). Sachs' ' Experimental Physiologie,' 1865, VII. 

 I established the foundation for an understanding of the transpiration-current as well as of the 

 excretion of water in the liquid state in my oft-cited treatise, ' Über die Porosität des Holzes' : 

 the obscurity prevailing in Pfeffer's ' Pflanzen-physiologie' (18S1), in the section on the move- 

 ments of water in the plant, might have been avoided if the author had suniciently regarded 

 this treatise. With respect to my observations on the Potato plant mentioned above, they are 

 tabulated in mv ' Experimental Physiologie' (p. 210). 

 [3] 



