148 PHYSIOLOGICAL BOTANY. 



propnate offices in the circulation; some conducting upward, 

 some downward, some conveying the crude sap, some latex, and 

 some air. 



470. Spiral vessels and others of the trachenchyma are gen 

 erally 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. 



471. 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 exhala- 

 tion and consequent exhaustion there going on. That tissue of 

 the stem and branches through which the ascending sap loves 

 chiefly to travel, is the pleurenchyma those long cells of the 

 wood fibre, whether arranged in broad layers, as in the Exogens, 

 or scattered in slender bundles, as in the Endoijens. And when 



' O 



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 albur- 

 num, next adjoining the liber. 



472. 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 



ncisions made in trees in early Spring. 



473. 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 

 Migar Maple, or sometimes spontaneously, as in the Grape. As soon as the buds expand 

 into leaves and flowers, the overflow ceases. 



474. 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 (here- 

 after 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. 



