30 SECONDARY PHLOEM 



Chestnut . . Castanea saliva (Europe) (constructional work). 



Elm . . Ulmus spp. (Europe) (coffin-boards). 



Hickory . Gary a spp. (N. America) (shafts, spokes, etc.). 



Jarrah, . . Eucalyptus marginata (Australia) (constructional work, 



wood-paving blocks, etc.). 



Lignum vitae . Guaiacum officinale (America) (shaft-bearings, etc.). 



Lime .. . Tilia spp. (Europe and N. America) (cabinet-work). 



Mahogany. . Swietenia mahagoni (West Indies, Peru) (cabinet-work). 



Maple , . . Acer spp. (Europe and N. America) (furniture). 



Oak . ; . Quercus spp. (Europe and America) (numerous uses, 



especially building). 



Poplar '' . . Populus spp. (Europe) (packing-cases, etc.). 



Sandal wood . Santalum album (E. Indies) (cabinet-work). 



Satinwood . Chloroxylon swietenia (E. Indies) (cabinet-work). 



Teak , . Tectona grandis (India, Java) (cabinet-work) . 



The secondary phloem, like the primary, is composed of sieve- 

 tubes, companion cells, and phloem-parenchyma, but these are 

 often accompanied by fibres. The septa in the sieve-tubes are 

 not uncommonly oblique and provided with several perforated 

 areas separated by bar-like thickenings (compound sieve-plates). 

 In many herbaceous plants, however (e.g. Vegetable Marrow, 

 Cucurbita) , the septum is horizontal and bears but one perforated 

 area. Functionless sieve-like areas are sometimes present as 

 isolated patches on the longitudinal walls of the sieve-tubes (cf . 

 Fig. 13, left). In some cases the phloem-parenchyma and sieve- 

 tubes, with their companion cells, are produced in such regular 

 sequence as to form alternating tangential bands. The fibres, 

 which show no distinctive features, are likewise often disposed 

 in layers separating the zones of thin-walled elements (e.g. Lime, 

 Mallow, etc.). Vertical series of phloem-parenchyma cells, occu- 

 pied by crystals of oxalate of lime, and elongated secretory 

 elements (e.g. tannin-sacs, cf. p. 151) are not uncommon. The 

 older secondary phloem which has passed out of use is, in the 

 case of woody plants, not infrequently shed with the bark (p. 140) . 



The parenchyma rays of the secondary phloem are continuous 

 with those of the xylem and exhibit the same general structure, 

 except that the component cells remain thin-walled. In certain 

 Flowering Plants (e.g. Lime, etc.) the outer ends of the primary 

 medullary rays exhibit a marked V-shaped enlargement, whereby 

 the secondary phloem becomes divided up into a number of 

 wedge-like groups. 



