244 



PLANT PHYSIOLOGY 



The pressure from below which results in bleeding is called 

 exudation or root pressure. That such pressure exists can be shown 

 by attaching a manometer to the cut stem as shown in Fig. 13, 

 although it should be added that the pressure is not an index to the 

 amount of sap which exudes. The pressure may be high and yet 

 there may be only a small amount of exudate, as the previous table 



shows. This pressure is a manifestation 

 of the turgor pressure in the cells below 

 the cut, which is apparently connected 

 with permeability changes in the cells, 

 with the result that water is forced out of 

 them into the vessels and up the stem; 

 but just how this turgor pressure is con- 

 nected with bleeding is still an unsettled 

 question. 



Glands. — Secretion is the exudation 

 from specialized cells called glands, and 

 is a common occurrence in plants. In ad- 

 dition to water, it is possible to find in 

 the secretions of various glands sugars, 

 oil, digestive enzymes, etc., some of which, 

 e. g., the enzymes, have obvious functions. 

 This is a good nutrient medium for many 

 Fig. 13.— Arrangement fungi, and the organism which causes pear 

 illustrating one type of blight frequently infects the plant through 

 manometer for measuring thig channel The seC retions from floral 

 root pressure. 



nectaries attract insects which bring about 



cross-pollination. Oily and resinous secretions may protect the 

 plant from predatory animals. Some of these secretions may be 

 only waste products. All, however, present the problem of how the 

 cell, which is normally impermeable to the secretions, permits 

 their escape. Does the turgor of the gland cell increase to such a 

 point that the materials inside are forced out? Or are there inter- 

 mittent changes in the permeability of the cell which permit the 

 contents to pass outside? In the light of recent investigations 

 the latter would seem the more probable solution. 



Guttation. — Guttation is the escape of water in drops from 

 uninjured plants. It generally occurs in leaves at the tips of the 

 veins, where there are especial outlets called water stomata or 

 hydathodes. These water losses occur under the same external 



