I40 PHYSIOLOGY OF NUTRITION 



occurring mostly when transpiration is retarded for some reason, as for example, 

 in the case of the Aroideae and other plants living in moist places (Fig. 80)." 



(b) Exudation Pressure. — The second condition determining the movement 

 of water in stems is the so-called root pressure, sap pressure, or exudation pres- 

 sure, which produces bleeding. This phenomenon was first investigated bv 

 Hales. 1 If a branch is cut from a grapevine in the spring, before the buds open, a 

 watery fluid is extruded from the wound. Hales bound a piece of animal bladder 

 over the cut end and found that the sap was excreted with such force that the 

 bladder was much swollen at first and was finally broken. To measure the force 

 with which the sap was extruded, Hales connected the cut end of a branch with 



Fig. 80. — Guttation from hydathodes Fig. 81. — Arrangement for measuring exu- 



at the edge of a leaf of Impatiens sultani. dation pressure. (After Pfeffer.) 



(After Pfeffer.) 



1880. Volkens, G., Ueber Wasserausscheidung in liquider Form an den Blättern höherer 

 Pflanzen. Jahrb. К. Bot. Gart. u. Bot. Mus. Berlin 2 : 166-209. 1883. Gardiner, Walter, 

 On the physiological significance of waterglands and nectaries. Proc. Cambridge Phil. 

 Soc. 5: 35-50. 1883. Wieler, A., Das Bluten der Pflanzen. Cohn's Beiträge zur Biol. d. 

 Pflanzen 6: 1-211. 1893. Haberlandt, G., Anatomisch-physiologische Untersuchungen 

 über das tropische Laubblatt. IL Ueber wassersecernirende und-absorbirende Organe. (I. 

 Abhandlung.) Sitzungsber. (math.-naturw. Kl.) K. Akad. Wiss. Wien юз 7 . 489-538. 

 1894. Idem, same title (IL Abhandlung.) Ibid. 104 7 : 55-11 6. 1895. Idem, Zur Kenntniss 

 der Hydathoden. Jahrb. wiss. Bot. 30 : 511-528. 1897. — Ed. 

 1 Haies, 1735- [See note i, p. 135.] 



G 11t tat i on, as Burgerstein terms this excretion of liquid water [see note e, p. 134], may be in- 

 duced in many plants by injecting the cut stem or petiole with water under pressure. A simple 

 way is to attach a cut branch, by a rubber tube (properly reinforced with cloth wrapping), to 

 the water-tap, having first filled the tube with water, and then to open the tap. Fuchsia, 

 Impatiens sultani, and Tropccolum majits (garden nasturtium) serve very well. This phe- 

 nomenon was first described by deBary (Bot. Zeitg. 27: 883. 1869). See also, for another 

 early description: Prantl, K., Die Ergebnisse der neueren Untersuchungen über die Spal- 

 töffnungen. Flora 55: 305-312, 321-328, 337-346, 369-382. 1872.— Ed. 



