284 



PLANT PHYSIOLOGY 



placed under it, the water exuding from the root system may be 



collected and analyzed (Fig. 84). This phenom- 

 enon has been called the ''bleeding" of plants. 

 The force that drives the water into the vessels, 

 the root pressure, is produced by live cells in an 

 active state that expend energy in the process. 

 To measure this force, a mercury manometer is 

 connected to the cut stump of the plant (Fig. 85). 

 The phenomenon of bleeding is exhibited by 

 various plants in different degrees. In some, like 

 Fuchsia, nettle, and sunflower, it is readily 

 detected. In others, for instance, the conifers, it 

 is ahnost imperceptible. Bleeding shows also 

 seasonal variations. It is most profuse in the 

 spring, especially in woody plants, when in some 

 of them, like the birch, maple, and grape, the 

 sap flows abundantly from the cut stems and 

 Fig. 85. — Meas- branches. 



uring root pressure ^ ^ pressure may be observed likewise in 



by means of a mer- ^ ♦^ 



cury manometer uninjured plants. Seedlings of cereals, when 

 (after Mohsch). 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. 86), Fuchsia, or 

 Alchemilla, and on the leaves of the 

 potato and buckwheat. This phe- 

 nomenon is known as ''guttation." 

 It is produced by the same root pres- 

 sure that drives the water through the 

 vessel of the leaf, just as through a 

 glass tube. Especially strong gutta- 

 tion takes place in Arum, Colocasia, 

 and Philodendron, plants commonly 

 cultivated indoors. Under favorable 

 conditions, such as high temperature 

 and high air and soil moisture, the 

 tips of their leaves may drip continually. 



The amount of water excreted under the influence of root 

 pressure is generally insignificant. It does not exceed a few 



Fig. 86. — Guttation of a nastur 

 tium leaf {after Noll). 



