204 PHYSIOLOGY OF NUTRITION. 



In summer we select rooms with a north aspect, in winter rooms 

 which can be heated with good stoves. 



1 See Detmer, Beitrdge zur Theorie des Wurzeldrucks, Jena, 1877, p. 30. See 

 also Wieler, in Cohn's Beitrdge zur Biologie der Pflanzen, Bd. 6, Pt. 1. 



2 See Pfeffer, Zeitschrift /. wissenschl. Mikroskopie, Bd. 7, p. 449. 



78. Periodicity of the Root Pressure. 



We decapitate pot plants of Cucurbita Melopepo about two 

 months old, or plants of Helianthus tuberosus * about the same 

 age, raised from tubers, or suitable examples of Primus Lauro- 

 cerasus. We fit up the stem-stumps with a simple tube or other- 

 wise, as described in 75, cover the soil in the flower pots with 

 tin-foil, to prevent excessive loss of water, introduce a thermo- 

 meter into the soil, and place the preparations in a room kept as 

 nearly as possible at a constant temperature, or transfer them to 

 a thermostat. If we now determine the quantity of sap escaping 

 during unit time (e.g. every hour or every two hours) at different 

 times of the day, we find that the plants, notwithstanding the 

 constancy of the external conditions, do not always yield the same 

 quantity of fluid. The flow of sap in Cucurbita and Helianthus 

 is most active shortly after midday, whilst in Prunus Laurocerasus, 

 according to my observations, the maximum outflow does not take 

 place till towards evening. During the night the flow diminishes. 

 It reaches its minimum in the early hours of the morning, and 

 then begins to rise again. If we have connected the stem-stumps 

 with a simple tube, as in Fig. 73, we must naturally, after each 

 observation, remove any fluid above the mark. This is con- 

 veniently effected by means of a thin glass tube. The length of 

 the column of water from the mark to the surface of the fluid, 

 expressed in millimetres, will serve as a measure of the quantity 

 of sap escaping. Comparatively young plants do not, as far as 

 investigations extend, exhibit the phenomenon of periodicity in 

 the flow of sap. For example, in plants of Cucurbita Melopepo 

 one month old, the periodicity has not yet developed. 1 It is ob- 

 vious that in all experiments special attention must be paid to 

 keeping the temperature as constant as possible. 



* The experiments with Cucurbita, Helianthus, and many other plants, may 

 be carried out in the summer. The plants must have been cultivated in a 

 sunny place in the open, or in the cold-house. 



