MOVEMENTS OF WATER IN THE PLANT. 



33 



25. Path of Sap Movements. The plant takes up water and 

 mineral salts from the soil, and forms foods from carbon diox- 

 ide in the leaves. These substances must pass from the roots 

 upward and from the leaf downward to be of use to the plant. 

 The ascending stream moves upward through the woody part 

 (xylem) of the stem. In trees the greater amount passes 

 FIG. 29. FIG. 30. 



Cross-section of portion of shoot of Cross-section of portion of stem 

 Sambucus nigra (Elder) magnified 15 of Sambucus nigra (Elder) 

 times. (After Oels.) ^, epidermis ; magnified 150 times. (After 

 k, cork ; rp, parenchyma and scleren- Oels.) rp, phloem paren- 

 chyma ; c, cambium ; ^, wood ; #//, chyma ; sc, sclerenchyma ; c, 

 pith. cambium ; h, wood ; m, me- 

 dullary rays. 



through five or six of the recently-formed annual rings, as may 

 be seen in trees with hollowed trunks which sustain tops of nor- 

 mal size. The descending current passes through the soft inner 

 bark, the phloem. The descending current moves very slowly, 

 and is carried on principally by diffusion (Figs. 29 and 30). 

 EXPERIMENT 39. 



UPWARD PATH OF SAP. 



Remove a ring of the bark and soft wood from any young 

 tree or woody shoot a few centimeters above the ground by means 

 of a sharp knife. The shoot shows no disturbance for a time vary- 

 ing from a few weeks to many months, when the roots become starved 

 from lack of food usually supplied by the leaves and perish. 



