126 PLANT PHYSIOLOGY 



movements. Nectaries are special organs on the surface of 

 which sugar is abundantly produced. What causes the for- 

 mation and excretion of sugar by the cells of nectaries is 

 not known, but given the sugar on the surface of a nectary, 

 the excretion of water to dissolve this is inevitable. * Why 

 this sugary solution is not resorbed is also unknown. Ap- 

 parently no clear idea of the action of nectaries can be had un- 

 til the physiological chemistry of these organs is worked out. 



The accumulation of resins in the intercellular resin-reser- 

 voirs of the Conifers, etc., and the incrustations of lime 

 and iron on the surfaces of various plants, are accounted 

 for in the following way. Certain substances elaborated or 

 formed as by-products by glandular cells, are excreted os- 

 motically into intercellular spaces, or upon the surface, or 

 under the cuticula (in certain hairs), there undergoing such 

 chemical change that they are no longer capable of osmotic 

 movement in water.! 



The excretion of fluid water by many plants is also ac- 

 complished by purely physical means. Water vapor is uni- 

 versally given off by land plants (p. 136), but the escape 

 of fluid water is a less frequent and regular occurrence. In 

 nectaries the passage of fluid water from the cell is due to 

 osmotic pressure, the attraction of the excreted sugar. In 

 other cases, on the contrary, fluid water is excreted, not 

 because of the attraction (osmotic suction) of substances 

 outside the cell but because of the pressure (turgor, p. 110) 

 within the cell. 



Turgor will develop in a cell whenever the cell-sap, be- 

 cause it contains a higher percentage of dissolved salts 

 than the water outside, can absorb water. The volume 

 of the cell-sap and of the enclosing protoplasm tends to 



* Wilson, W. The cause of the excretion of water on the surface of necta- 

 ries. Untersuchungen aus dem Bot. Institut zu Tiibingen, Bd. I., 1881. 

 Schimper, A. F. W. Wechselbezienhungen zwischen Pflanzen und Ameisen, 

 1888. 



i See Pfeffer, Pflanzenphysiologie, Bd. I., pp. 115, 116 501; Engl. transl., 

 pp. 129, 500. Kohl, F. G. Kalsalze und Kieselsaure in der Pflanze, 1889. 

 Giesenhagen, C. Die radialen Strange der Cystolithen von Ficus elastica. 

 Berichte der Deutsch. Bot. Gesellsch., 1891. Tschirch, A. Die Harze und 

 die Harzbehalter. Berlin, 1900. 



