446 ELEMENTARY BIOLOGY. [CHAP. 



colour ; but the thickening has taken place unequally, so as 

 to leave short, obliquely directed, thin places, which look 

 like clefts and are called pits. This tissue is termed scleren- 

 chyma. The yellow bands, lastly, are vascular bundles. 

 Each bundle is surrounded on the outside by a layer of 

 rather thick-walled, elongated, parallel-sided cells, con- 

 stituting the bundle- sheath. The bundle itself consists of 

 two main parts ; a central portion, constituting the xylem or 

 wood, and a peripheral portion which is the phloem or bast. 

 Bundles of this structure, in which the xylem is surrounded 

 by the phloem, are termed concentric. The xylem consists 

 chiefly of vessels, many of them of relatively large size. They 

 are derived from cells, the transverse walls between which 

 have been partially broken down. In the mature vessels 

 the protoplasmic contents have entirely disappeared ; they 

 only contain water or air. Their walls are greatly thickened, 

 the thickening having taken place along equidistant trans- 

 verse lines, the thin spaces left between them being the 

 pits. The vessels have become flattened against one another, 

 by mutual pressure, so that they are five- or six-sided ; and, 

 as the markings of their flattened walls simulate the rounds 

 of a ladder, they have been termed scalariform ducts or 

 vessels. The cavities of these scalariform ducts are divided 

 at intervals, in correspondence with the lengths of the cells 

 of which they are made up, by oblique, often perforated, 

 partitions. Among the smaller vessels, a few will be found, 

 in which the thickening forms a closely wound spiral. 

 These are spiral vessels. They usually occur in two groups, 

 and as they are the first elements of the xylem to be differ- 

 entiated, they are said to constitute the protoxylem of the 

 bundle. Among the vessels a few parenchymatous cells 

 containing starch are scattered. The phloem is at once dis- 

 tinguished from the xylem by the smaller average si/e of its 



