154 CIRCULATION. 



offices in the circulation, some conducting upward, some downward, some 

 conveying the crude sap, some latex, and some air. 



790. AIR-VESSELS. Spiral vessels and others of the trachenchyma 

 are generally filled with air, and take no part in the circulation of fluids, 

 except in the spring, when the whole system is gorged with sap. The 

 intercellular passages, also, generally circulate air alone. 



791. THE MOVING FORCE. From the roots the newly absorbed fluid 

 flows upward through the stems and branches, toward the buds, leaves, 

 and flowers, being probably drawn thither into them by the exhalation 

 and consequent exhaustion there going on. 



792. THROUGH WHAT TISSUE. The tissue of the stem and branches 

 through which the ascending sap loves chiefly to travel is the pleuren- 

 chyma those long cells of the wood fiber, whether arranged in broad 

 layers, as in the Exogens, or scattered in slender bundles, as in the En- 

 dogens. 



793. THROUGH WHICH LAYERS. And when the stem grows old, the 

 sap ceases to traverse the inner layers, the duramen, where its passage 

 becomes obstructed by thickened cell walls, and frequents only the outer 

 newer layers, the alburnum, next adjoining the liber. 



794. THE CRUDE SAP. The fluid which thus flows upward seeking 

 the leaves consists largely of water, is colorless, and is called the crude 

 sap. It contains in solution minute quantities of gases and mineral 

 salts, imbibed by the roots, together with dextrine and sugar (no starch) 

 which it dissolved out of the cells on its way. This is the fluid which 

 flows so abundantly from incisions made in trees in early spring. 



795. THE OVERFLOW OF THE SAP depends upon the excess of absorption over 

 exhalation. After the decay of the leaves in autumn, and the consequent cessation 

 of exhalation, the rootlets, being deep in the ground, below the influence of frost, 

 continue their action for a time, and an accumulation of sap in the system, even in 

 the air-vessels and spaces, takes place. Also in early spring, before the leaves are 

 developed, this action recommences, and the plant becomes gorged with sap, which 

 will burst forth from incisions, as in the sugar maple, or sometimes spontaneously, 

 as in the grape. As soon as the buds expand into leaves and flowers, the overflow 



796. THE TRUE SAP. Throughout its whole course to the leaves the 

 sap gains in density by solution. There arrived, it loses by exhalation 

 a large part of its water, gains additional carbon, and undergoes other 

 important chemical changes (hereafter to be noticed), and becomes the 

 true sap, dense and rich, both in nutritive matter for the immediate 

 growth and in special products for the future nourishment of the plant. 



797. RETURNING, the true sap distributes its treasures in due and ex- 

 act proportion as needed to every organ. Its course lies in the tissues 

 of the bark, cellular and woody, first distributed over the under surface 



