AS AIDS IN THE DETEBMINATION OF SPEGIES. 15 



The wood fibres, forming mainly the outer half of the woody cmwe, are 

 recognised by their thickened walls and comparatively small cavity. They are on an 

 average l-3000in, in diameter. 



The wood parenchyma consists of small elongated cells, arranged in radiating 

 lines so as to resemble medullary rays. They stain of a dark colour, and hence stand 

 out clearly from the surrounding yellow. The radiating lines are usually just 

 sufficiently equidistant to allow room between for a radiating row of vessels. The 

 average length of each cell is about l-2000in. 



The vessels, conspicuous by reason of their large cavities, are arranged in radial 

 rows, and form mainly the inner half of the woody curve. A larger or smaller number 

 of vessels generally appear on the outer border, at the central and thickest portion of 

 the wood. The diameter of the vessels varies considerably, the largest being about 

 1-750 in., and the average diameter about half that. In a longitudinal section the 

 vessels are seen to be spiral and annular. 



From the foregoing brief description of the parts of the F. V. B. in the petiole 

 of E. globulus, it will be seen that the bundle here is different from the normal 

 bundle. Usually the phloem and xylem follow each other in the same radius, the 

 phloem being outside and the xylem inside, so that when a bundle is traced into the 

 leaf stalk, the phloem portion is towards the outer or lower surface, and the xylem 

 portion towards the inner or upper surface. In this instance, however, there is 

 phloem not only on the outer but also on the inner side of the wood, and continued 

 round, so that the wood is in the centre completely surrounded by the phloem. In 

 most cases the phloem forms a narrow band, where it curves round to become 

 continuous. Such a bundle, with the xylem completely surrounded by the phloem, is 

 called concentric. With the normal arrangement of phloem and xylem, the form of 

 the bundle is collateral, and when there is phloem outside and inside the xylem it is 

 distinguished as bicollateral. According to De Bary,* the latter is the form of the 

 bundle in all investigated species of Eucalyptus, and " Eucalyptus globulus decidedly 

 belongs to this series." As far as the petiole is concerned in this, and we may add, 

 in the other species investigated, the bundle belongs to the concentric type. 



Having seen the mode of arrangement of the parts of the F. V. B., we will now 

 endeavour to explain the use of this arrangement and structure. 



The hard bast all round is evidently, from its very nature and position, a 

 mechanical support and protection. 



The wood fibres in the centre likewise play the part of mechanical support, and 

 the soft bast is safely placed between the hard bast and the inflexible wood. 



The cambiform cells of the soft bast have thin walls, to allow of the circulation of 

 diffusible substances, but as the thin walls might collapse and thick walls would pre- 



* Comp. Anat. of Phanerogams and Ferns, p. 338. 



