PHLOEM AI^D XYLEM 13 



chloride, potassium-iodide, and iodine which turns unaltered 

 cellulose blue. 



The elements of the phloem, with which we are less concerned 

 than we are with the xylem, though often variously thickened, 

 are not Hgnified. They consist of last-parenchyma, sieve-tubes, 

 companion-cells, and bast-fibres, besides the medullary rays which 

 traverse xylem and phloem ahke. Bast-parenchyma consists of 

 sHghtly elongated cells in vertical rows of four or six, of which 

 the terminal cells taper. This arises from each row having been 

 formed by several transverse divisions of a single prqcambium or 

 caxnbiuzn eeU. They generaUy contain protoplasm and sometimes 

 grains of starch or crystals. Sieve-tubes are the vessels of the bast, 

 long tubes with transverse partition-waUs, and retaining their 

 protoplasm but communicating through these transverse walls by 

 the sieve-plates from which they take their name. The sieve-plate 

 is a thin portion of the wall perforated by numerous pits close 

 together. The sieve-tubes are the chief channel by which proto- 

 plasmic matter manufactured in the leaves is conveyed through 

 the stem. Companion-cells occur only in angiosperms. In longi- 

 tudinal section they appear as narrower cells alongside the sieve- 

 tubes Med with granular protoplasm and with unperforated 

 transverse walls adjoining those of the sieve-tubes. In a trans- 

 verse section they appear hke small corners cut off the larger 

 sieve-tubes, and they have their name from the fact that each of 

 them originates in this way, a longitudinal wall dividing the 

 original cell into two unequal parts, of which the larger contributes 

 to a sieve-tube, the smaller remains a cell. Bast-parenchyma, 

 sieve-tubes, and companion-cells are known collectively as soft bast 

 m contradistinction to bast-fibres or hard bast. Bast-fibres are 

 extremely elongated structures, tapering at each end, containing 

 only water or air, and with their walls so thickened as sometimes 

 to almost obhterate the cavity or lumen, as it is termed. Their 

 walls are generally at least partially lignified and give a reddish 

 colour with Schulze's solution, and the thickening is absent from 

 some spots on their walls. These unthickened spots are known 

 as pits. Pits, which are important as occurring also on some of 

 the elements that make up wood, are of two main classes, simple 

 and bordered. A simple pit is a spot at which a cell-wall is left 

 unthickened, generally on both sides, each successive thickening- 

 layer leaving the same space uncovered. It appears accordingly 

 as a bright spot on the wall ; or, if in section, as a canal, the 

 length of which depends upon the thickness of the waU. A 

 bordered pit is so called because the bright spot appears surrounded 

 by, or crossed by, a second circle or ellipse. The structure will 



