82 PHLOEM 



In the phloem many of the large somewhat thick -walled 

 sieve-tubes appear empty ; but here and there, where the plane 

 of section coincides with a separating wall, there is a deeply 

 stained sieve-plate (Fig. 13, s.p.), recognisable by its dotted 

 structure. Under the high power the sieve-plate exhibits an 

 irregular network of thickening, completely covered by a thin 

 layer of cytoplasm, the greater part of each mesh being occupied 

 by an open pore through which communication with the next 

 segment of the sieve-tube is established. In contact with each 

 sieve-tube are one or more small, often more or less triangular 

 companion cells (Fig. 13, c.c.), with dense contents. The 

 numerous phloem-parenchyma cells (Fig. 13, p.) are more or 

 less intermediate in size, but without either the thickened wall 

 of the sieve-tube or the dense contents of the companion cell. 

 Both inner and outer phloem show the same structure. 



In longitudinal sections (Figs. 13, left, and 38) the horizontal 

 sieve-plates (s.p.), stained red with the eosin, occur at frequent 

 intervals in the course of the sieve-tubes. When viewed in 

 optical section, they present an interrupted appearance, due to 

 the alternation between the pores and the bars of thickening 

 with their covering of cytoplasm. Thick, highly refractive masses 

 (stained red by the eosin), composed of a carbohydrate known as 

 callose, are often found on one or both sides of the sieve-plates. 

 Such deposits of callose, bringing about a closure of the latter, 

 arise sooner or later in the sieve-tubes of most plants, and are 

 often permanent, marking the end of the activity of the element 

 in question ; in some woody plants and in many Monocotyle- 

 donous rhizomes this is, however, a periodic phenomenon, the 

 callose being formed in the autumn and becoming redissolved 

 at the advent of spring. Callose is distinguished by being 

 insoluble in ammoniated copper oxide, but soluble in a I per cent, 

 solution of caustic potash, and by its reddish-brown colouration 

 with chlor-zinc-iodide ; it becomes deeply stained by a dilute 

 aqueous solution of aniline blue, which should be allowed to act 

 for half an hour. 



Here and there the plane of the section passes through a 

 companion cell (Fig. 13, c.c.) in contact with its sieve-tube. The 

 former tapers off towards the sieve-plates above and below, so 

 that its length coincides with that of the sieve-tube segment 



