234 



BOTANY 



PART I 



the main physiological part in the process as in the case of the 



secretions of the NECTARIES, of the DIGESTIVE GLANDS of insectivorous 



plants (p. 258), and of the 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 'and tracheides, and is 

 pressed out with considerable force (ROOT 

 PRESSURE). 



If a long glass tube be placed on the root- 

 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. 243). The column 

 of mercury will in some cases be forced to a 

 height of 50 or 60, and under favourable con- 

 ditions 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 greater when the soil is kept 

 moist and warm ; it continues under such con- 

 ditions, according to the kind of plant and its 

 stage of development, some days or even months. 

 f The water may amount to many litres : up to 



FIG. 243. Vigorous exudation 



water as the result of root-pres- l litre per day in the V i ne , 5 litres in the Birch, 

 sure from a cut stem of Dahlia. , , ., _, -,.. 17 . _. . 



The smoothly-cut stem sis joined and 1Q - 15 lltres m_ Palms. In parts of plants 



to the glass tube g by means that continue bleeding for some time a certain 



of the rubber tubing c. The water periodicity in the amount is noticeable ; more is 



W, absorbed by the roots from extruded by night than by day. 



The outflowing sap often contains, in addition 



the soil, is pumped out of the 

 vessels of the stem with a force 



sufficient to overcome the resist- to mineral salts, considerable quantities of organic 



ance of the column of mercury Q. 

 (After NOLL.) 



substances (dissolved albuminous matter, as- 

 paragin, acids, and especially carbohydrates). 

 The amount of saccharine matter in the sap of 



some plants is so great that sugar may be profitably derived from it. The sap of 

 the North American sugar maple, for example, contains J per cent of sugar, and a 

 single tree will yield 2-3 kilos. The sap of certain plants is also fermented and 

 used as an intoxicating drink (birch wine, palm wine, pulque, a Mexican beverage 

 made from the sap of Agave, etc.). One inflorescence of Agave will yield 1000 

 litres of sap in from four to five months. 



Causes of the Excretion of Water ( 17 ). The excretion of drops of water from 

 intact plants is in part due to an active excretion of water from superficial cells. 

 In other cases water is forced into the vessels, and finds a way out at the points of 



