FLUID PARTS OF VEGETABLES. 143 



to the purpose of producing changes in the fluids of the plants ; 

 thus the sap is converted into the proper juices, and from the 

 same soil and nourishment appear plants of very different pro- 

 perties. 



Mirbel, by the aid of the microscope, succeeded in discovering 

 a system of glands, in the pores of cells, and on the borders of 

 the spiral vessels. There are external glands which appear 

 manifest to the naked eye ; as the nectaries of flowers, which 

 secrete or manufacture honey ; and the stings of plants, which 

 secrete an acrid substance, that by penetrating the skin causes 

 a painful sensation. 



Fluid parts of Vegetables. 



The different fluids which are exhibited in the vegetable 

 body may be considered under three general divisions : 1st, the 

 sap, or ascending fluid ; 2d, the cambium, or descending fluid ; 

 3d, the proper juices. 



The sap is a limpid, inodorous liquid, the elements of which 

 are imbibed from the earth by pores in the radicles of the root. 

 You know that if the earth around the roots of plants is depriv. 

 ed of moisture, they soon die. Moisture furnishes to the radi- 

 cles, water holding in solution various substances ; such as 

 earths, salts, animal and vegetable matter. The radicles, by 

 some unknown process, convert this fluid matter into sap, and 

 then, by means of vessels which form what is called the sap- 

 wood or alburnum, this sap ascends through the stems to the 

 branches ; passing through the woody part of the petioles, and 

 those minute branches of the petiole which form the ribs and 

 veins of the leaf, it enters into the vessels and cells which ex- 

 tend throughout its substance. 



The ascending sap is always in circulation, but its energy 

 varies with the season and the age of the plant. Heat has an 

 important influence upon the ascent of the sap; yet during a 

 dry and hot season it often appears to ascend but slowly. This 

 is because the absorption of fluids from the earth is checked by 

 the dryness of the soil. The plant, by a little stretch of the 

 imagination, may be considered as thirsty, and thus man may 

 seem not only provident, but humane, in administering to its 

 roots refreshing draughts of water. Even the leaves, at such a 

 period, seem to be too impatient to wait for supplies by means of 

 the connecting sap vessels ; but if water is sprinkled upon 

 them, they fail not to use their own power of absorption, and 

 may, upon such an application, be seen to revive almost in- 

 stantaneously. 



Their use Mirbel's discoveries External glands '1 hree divisions of fluids 

 Sap Sap-wood Ascent of the sap. 



