20 PRINCIPLES AND PRACTICE OF PRUNING 



materials in the sap-wood and tlic inner )iark. The increase in the 

 wood ring, which accompanies tliem, is almost nil. These growths, 

 however, are of economic importance, since they are the means of 

 removing the starchy compounds from the timljer — compounds which 

 indirectly do much to hasten decay. It may be that the reputation 

 of winter-felled timber is partly due to this fact. 



24. "Phloem, that portion of a fibro-vascular bundle in 

 plants containing the bast and sieve tissue. Tn exogens 

 it is ahvaj's sharply defined from the remaining portion 

 (xylem, 25) by a layer of cambium. The inner bark is 

 derived from the phloem, the wood from the xylem. The 

 elaborated plant food from the leaves passes down and 

 is distributed by the phloem. 



25. "Xylem, or woody portion of a fibro-v^ascular 

 bundle which contains the larger continuous air-holding 

 vessels and the walls of whose cells are often thickened 

 and lignified. The xylem is separated from the phloem 

 by the cambium, when there is any, and it usually oc- 

 cupies the side of the bundle toward the center of the 

 stem. Water with the mineral compounds in solution 

 passes up through the xylem to the leaves." * 



26. Fibro-vascular bundles. — The conducting tissue 

 discussed above is arranged in fibro-vascular bundles. 

 In monocot_\ledonous plants (sorghum, corn) these bundles 

 are distributed irregularly through softer tissue (paren- 

 chyma). Because of this irregular distribution, and also 

 because there is no cambium within the bundles, mono- 

 cot}ledonous jilants usually have neither true bark, cambium 

 rings nor annual rings of wood. In dicotyledonous plants, 

 on the other hand, the fibro-\'ascular bundles have a cam- 

 bium which separates the xylem from the phloem. In 

 dicotyledonous plants, which form a wood-ring, the primary 

 tissue of the bundles forms a layer in which the outer 

 part contains the phloem and the inner the xvlem. Since 

 the cambium is between these two sets of tissue, new 

 xylem and phloem are (le\e1oped as growth proceeds; 



• Kains. Plant Propagation, Grecnhoire onj Nursery Practice, Paj;e 146. 



