19 



these special cells occur largely in the mesophyll, although gradually diminishing 

 in number till C. Macleayana is reached, when they appear in both forms of the 

 mesophyll, and most pronounced amongst the parenchymatous cells in the neigh- 

 bourhood of the leaf bundle and oil cavity, which mode of occurrence adds another 

 evidence of the isolation of this species from its congeners. The principal terpene 

 in the leaf oil of this group is pinene. 



From these observations it would appear that there exists some connection 

 or agreement between these bodies which go to make up the leaf substance, and 

 the chemical constituents of the oil, and it is scarcely to be expected that those 

 Callitris species, whose leaves give an oil in which the dextro-rotatory limonene 

 predominates are identical in all their structural characters with those species in 

 which the predominant limonene is Isevo -rotatory. The maxima of the rotations 

 of the limonenes in these trees are reached by slow gradations through the 

 several species, and this evidently indicates constructive peculiarities in the leaf 

 arrangement, even if not in the structure of the whole tree. 



The method by which living plants construct the various asymmetric 

 chemical substances is at present practically unknown, but there seems no reason 

 why systematic study in this direction should not eventually be rewarded with 

 as satisfactory a result as has been the investigation of the asymmetric compounds 

 themselves. In whatever direction the forces of nature have exerted their 

 influence in the construction of these active forms, it can hardly be without 

 leaving a corresponding impression upon the plant tissue itself, so that a close 

 connection between the chemical and botanical phenomena of the leaves of the 

 several species of Callitris must be present, and its identification is here attempted. 

 To successfully trace the evidence leading to the selective formation of these 

 asymmetric terpenes in the several species of the Callitris would add considerably 

 to our knowledge in this direction. Since the time when Pasteur advanced his 

 views upon this question of optical activity, a considerable amount of work has 

 been undertaken in the endeavour to add to our knowledge in this direction, 

 but, so far, with no very certain results.* 



There appears to be a considerable break, both botanically and chemically, 

 in the sequence connecting C. Macleayana with the other Callitris, and this 

 " Stringybark Pine " is evidently located at the end more nearly approaching 

 the Arattcarias. Whether Araucaria is the older genus or not, there is at present 

 insufficient evidence to decide, but the distance separating these two genera is 

 not great. They are both of considerable age on this continent. 



* References to much of this work, together with a bibliography, are given by A. W. Stewart in his work on 

 stereo-chemistry, London, igoy. We would also direct attention to the address on this subject by Professor F. R. 

 Japp, before the British Association in 1898, and the subsequent criticisms thereon, published in " Nature." Vols. 58 

 and 59 



