123 



III. LEAVES. 



(rt) Economics. 



The presence of the oil is of course the main economic product of these 

 organs. As a fodder plant they have little to recommend them, for it is only 

 during the severest drought that sheep will nibble them, and then not for long. 



(b) Anatomy. 



For descriptive purposes cross sections were taken near the end of a 

 branclalet and at various intervals along the decurrent portion of the leaves. 

 Such sections were found satisfactory for histological work, for they included 

 one part of each decurrent leaf as well as the portion of the branchlet which 

 formed the central column to which the leaves were attached, the whole giving 

 a well-delined trefoil in shape. 



The free portion of the leaf was of little value in working out the anatomical 

 structure of this part of the plant as obtains in the needles of Piniis, where a group 

 of vascular bundles forms the central column around which regular leaf tissue is 

 sustained, whilst in Callitris the ultimate portion of the branchlet composes the 

 central vascular sj-stem supporting adnate leaf sections which collectively appear 

 to form one whole, or at least that is the ^•iew here taken of this part of the 

 tree for descriptive purposes. 



The central xylem of the branchlet is succeeded b\- a normally orientated 

 phloem ; the relative position of these elements, therefore, is in accord with their 

 final disposition in maturity of stem and branches. Subsidiary to these \nll be 

 found near the base of each concrescent division and next the oil gland a small 

 bundle ^a primary leaf bundle, so to speak) of the true leaf, with the phloem 

 normally orientated; these and the central bundle might perhaps be considered 

 as corresponding to the median and secondar}- bundles of an ordinary bilateral 

 leaf. 



The xylem and phloem cells call for no special remark, as they conform to 

 the usual characters of such found in the vegetable kingdom. 



The phloem of the central system of the branchlet is surrounded b}^ a mass 

 composed of (i) parenchymatous endodermal cells ; (2^ transfusion tissue : — 

 the tracheids of which in the case of this and other species of Callitris appear to 

 have no uniformity of arrangement when the section is taken either through, 

 or clear of, the oil ca^ities, as against the uniformit}- of such found in most other 

 Conifers. When, however, oil ca\ities are present, the parenchymatous, or what 

 may perhaps be regarded in this case as the endodermal cells, are found to extend 

 round and encircle these bodies, and also to form a group or cluster between the 

 central axis and the epidermis at the base of the cavity formed by the concave 



