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TEXTBOOK OF PLANT PHYSIOLOGY 



most profuse in spring, especially in woody plants, when in some 



Fig. 53. — Exudation of sap from a cut stem as a result of root pressure (after 



Molisch) . 



of them, as the birch, maple and grape, the sap flows abundantly 

 from the cut stems and branches. 



Root pressure may be observed likewise in 

 uninjured plants. Seedlings of cereals, when 

 placed in a moist atmosphere, accumulate water 

 drops on the tips of their leaves. From time 

 to time these drops run down and are replaced 

 by new ones. Similar drops may be observed 

 at the margins of leaves of nasturtium (Fig. 

 55), Fuchsia, or Alchemilla, and on the leaves 

 of the potato and buckwheat. This phenomenon 

 is known as " guttation." It is produced by 

 the same root pressure that drives the water 

 through the vessel of the leaf, just as through 

 a glass tube. Especially strong guttation may 

 be observed in Arum, Colocasia, and Philo- 

 dendron, plants commonly cultivated indoors. 

 Under favorable conditions, such as high tem- 



Fig. 54. — Measuring ' ° 



root pressure by perature, and high air and soil moisture, the tips 



means of a mercury Q f their leaveg ma dri continually. 



manometer (after J J 



Moloch). The amount of water excreted under the 



influence of root pressure is, on the whole, 



insignificant. It does not exceed a few cubic centimeters per day. 



