128 



PHYSIOLOGY OF NUTRITION 



occurring mostly wher 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 by- 

 Hales.^ 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. — Gitttation from hydathodes 

 at the edge of a leaf of Impatiens sullani. 

 {After Pfeffer.) 



Fig. 81. — Arrangement for measuring ex- 

 udation pressure. (After Pfeffer.) 



1 Hales, 173s. [See note i, p. 123. ] 



1880. Volkens, G., Ueber Wasserausscheidung in liquider Form an den Blattern hoherer 

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

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

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

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

 uber das tropische Laubblatt. II. Ueber wassersecemirende und-absorbirende Organe. (I. 

 Abhandlung.) Sitzungsber. (math.-naturw. Kl.) K. Akad. Wiss. Wien 103^- 489-538. 

 1894. Idem, same title (II. Abhandlung.) /ftid. 104': 55-116. 1895. Idem, Zur Kenntniss 

 der Hydathoden. Jahrb. wiss. Bot. 30 : 511-528. i8gy.—Ed. 



' Gultalion, as Burgerstein terms this excretion of liquid water [see note e, p. 1 2 2] , may be in- 

 duced in many plants by injecting the cut stem or pe tiole 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 suUani, and Tropmolum majus (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 iiber die Spal- 

 teffnungen. Flora 55: 305-312, 321-328, 337-346, 369-382. 1872.— £d. 



