LIFE OF THE WHOLE PLANT. 



797 



2. Distribution of Fluid Matters throiigh the Plant, and thiir 

 Alteration in the Leaves. — The fluid matter thus absorbed by 

 the roots is carried upwards by their tissues {jig. 1125) to the 

 stem, and through its young portions to the leaves, &c. (as 

 indicated by the arrows in the figure), to be aerated and elabo- 

 rated. After this it is returned to the stem, and descends probably 

 by the inner bark and cambium layer of Dicotyledons towards 

 the roots from which it started ; and by means of the medullary 

 rays and the general permeability of the tissues of which plant's 

 are composed, it is distributed to their different parts where new 

 tissues are being formed, and where secretions are to be deposited. 

 This general distribution of the fluid matters through the plant 

 is commonly termed the circulation of the sap. The fluid as it 

 ascends is called the Ascending or Crude Saj), and as it de- 

 scends, the Descending or Elaborated Sap. Although the term 

 Circulation is thus commonly applied to this movement of the 

 sap, it must be borne in mind, tha,t the process bears no 

 analogy to the circulation of the blood in animals ; for plants 

 have no heart or any organ of an analogous nature to propel 

 their fluid matters, nor any system 

 of vessels in which a flow thus 

 produced takes place. As Professor 

 Johnson has well put it, " nutrient 

 substances in the plant are not 

 absolutely confined to any path, 

 and may move in any direction. 

 The fact that they chiefly follow 

 certain channels, and move in this 

 or that direction, is plainly de- 

 pendent upon the structure and 

 arrangement of the tissues, on the 

 sources of nutriment, and on the 

 seat of growth or other action." 



Ascent of the Sap. — The sap in 

 its ascent to the leaves, &c., passes 

 principally through the yoimg un- 

 incrusted wood-cells aud vessels 

 (pp. 739 and 740), and therefore in 

 Dicotyledons, when they are of any 

 age, through the outer portion of 

 the wood or the alburnum. In such 

 plants, also, we have but one 

 main stream of ascending sap. In 

 Monocotyledons and Acotjledonous 



stems, the ascent also takes place through the unincrusted cells 

 of the fibro-vascular bundles; and hence in such plants, and 

 more especially in Monocotyledons, we have a nimiber of more 

 or less distinct ascending streams. In the lower Acotyledons, 



Fig. 



1125. Diagrammatic section of 

 the stem of a Dicotyledon, show- 

 ing the distribution or circulation 

 of the sap. The direction is indi- 

 cated by the arrows, a, a. Roots, 

 by which the fluid matters are ab- 

 sorbed, b, b. The tissues by which 

 they ascend to the leaves, c, c. 

 d, d. Outer portions of stem and 

 bark where the descent takes 

 place, e. Vertical section of a 

 branch. After Balfour. 



