SECT. II 



PHYSIOLOGY 



201 



the conditions are unfevourable to transpiration, especially in sub- 

 merged water plants, it may in a sense take the place of transpiration 

 in maintaining the current from the water-absorbing organs. Its 

 physiological significance is not, however, the same as transpiration, 

 since the expressed water always contains salts, and sometimes also 

 organic substances in solution. In fact the quantity of salts in water 

 thus exuded is often so abundant that 

 after evaporation a slight incrustation is 

 formed on the leaves (the lime-scales on 

 the leaves of Saxifrages). In some in- 

 stances, also, the substances in solution in 

 the water may play the main physiological 

 part in the process as in the case of the 

 secretions of the nectaries, of the diges- 

 tive GLANDS of insectivorous plants, and 

 of the viscid stigmatic fluid. 



Bleeding (^^). — Exudation of water 

 may often be observed after a plant has 

 been wounded ; it is regularly seen in 

 trees and shrubs when cut in the spring, 

 and is especially well marked in the Vine. 

 In shrubs cut off a short distance above 

 the ground, the extrusion of water from 

 the wound is readily demonstrated. In 

 this weeping or bleeding of wounds the 

 water comes from the vessels andtracheides, 

 and is pressed out with considerable force 

 (root-pressure). 



If a long glass tube be jilaced on tlie root- Fig. 188.— Vigorous exudation of 



water as the result of root-pres- 

 sure from a cut stem of Dahlia 

 variahilis. The smoothly cut stem 

 s is joined to the glass tube gr by 

 means of the rubber tubing c The 

 water ir, absorbed by the roots 

 from the soil, is pumped out of 

 the vessels of the stem with a force 

 sufficient to overcome the resist- 

 ance of the column of mercury Q. 



stump and tightly fastened by rubber tubing, the 



exuded fluid will be forced up the glass tube to a 



considerable height. How great the force of this 



pressure is may be shown by attaching to the 



stump a manometer (Fig. 188). The column of 



mercury will in some cases be forced to a height 



of 50 or 60, and under favourable conditions to 



140 cm. or more (in the Birch). These pressures 



would be sufficient to raise a column of water 6, 8, 



and 18 metres high. The amount of water extruded is gi-eater when the soil is kept 



moist and warm ; it continues under such conditions, according to the kind of plant 



and its stage of development, some days or even months. The water may amount 



to a litre or more : up to 1 litre per day in the Vine, 5 litres iia the Birch, and 



10-15 litres in Palms. In parts of plants that continue bleeding for some time a 



certain periodicity in the amount is noticeable ; more is extruded by night than 



by day. 



The outflowing sap often contains, in addition to numerous salts, 

 considerable quantities of organic substances (dissolved albuminous 



