12 THE TEANSVEKSE SECTIONS OF PETIOLES OF EUCALYPTS 



vessels (Fig. 5). The soft bast was carefully examined, and found to consist of 

 cambiform cells and sieve-tubes (Figs. 6 and 7). The cambiform cells varied 

 considerably in shape and size. Often they were brick-shaped, and even somewhat 

 roundish ; at other times they were much more elongated and thinner- walled. 

 Directly adjoining the wood the cells were clearly marked off from the latter, and 

 sufficiently thin-walled in some cases to resemble cambium. 



The sieve-tubes are shown in longitudinal section. (Fig. 7.) The iron stain, 

 as well as Schulze's solution, serves to differentiate them. 



FiBRO- VASCULAR BuNDLE. 



This is a convenient collective term for the group of tissues already separately 

 mentioned as hard bast, soft bast, and wood tissue, or if the bast and wood be 

 simply contrasted, then phloem and xylem may be used. 



Since the bundles form the most striking feature of these sections, and are the 

 parts from the form of which the species of Eucalypts are chiefly determined, they 

 will be expected to receive a large share of attention and illustration. 



The special form of the bundles, or rather of the woody portion of them, will be 

 considered in the next section devoted to the determination of species, so little will 

 be said here upon that point. 



It is to be premised, first of all, that the xylem portion of a leaf bundle is, as a 

 rule, towards the upper surface, and the phloem portion towards the lower surface, 

 just as would naturally happen in a lateral expansion of these tissues of the stem. 

 Also, that the upper or inner surface of the petiole has often a different contour to 

 the lower or outer surface. In this instance the upper surface is flatfish, while the 

 under surface is rounded. But in cases such as the fig and walnut, where the petiole 

 is cylindrical, the bundles are likewise arranged in a circle, and form a cylinder. 



As is well known, in most Eucalypts the blade of the leaf is not spread out 

 horizontally, as is usually the case, but vertically in relation to the leaf-bearing axis. 

 In the young condition, however, it has the normal position, and it is by subsequent 

 twisting of the leaf stalk that the changed position is brought about. In the course 

 of this twisting process, stomata are developed on the upper surface, in addition to 

 those already existing on the under surface, and palisade parenchyma is developed 

 beneath the lower epidermis as well.* But it appears that the bundle of the leaf 



* Bower and Vines, Practical Botany, Part I., 2nd Ed., p. liS. 



