23 



Macroscopically the timbers of the respective species present no charac- 

 teristic features that render identification easy under all circumstances, and 

 microscopically, also, there appears to be only minor points of differences. 



Evidences of the geological age of the genus are not, so far, very many, and 

 what there are rather point to an origin probably older than the Avaucarias, 

 and we are inclined to think that further palaeontological researches will reveal a 

 much older age than that now assigned to the genus. Ettingshausen (" Tertiary 

 Flora of Australia," p. go, pi. viii) records it under C. prisca, from Vegetable Creek, 

 Emmaville, New South Wales, in the Tertiary Period. According to Masters, 

 Unger records it as Eocene. 



V. FOLIATION. 



After the cotyledons burst forth through the testa, the plumule gives place 

 to a cluster of small pyram'dal-shaped leaves which may be classed as primordial ; 

 these, with the growth of the central stem, are developed at diminishing intervals 

 in whorls of threes, having a maximum length of ij inch, and it is this characteristic 

 leaf that obtains during this period of the life history of practically all the species 

 of the genus. 



When the young plant has grown to the size of 3 or 4 inches, and as the stem 

 develops, the length of the leaves appears to become less in each whorl, but this 

 diminution in length is rather apparent than real, for it is not that the leaves are 

 so much shorter, but that a much larger proportion of the leaf has become 

 decurrent or concrescent on the central stem. 



The normal leaf has, therefore, a very large proportion of its length running 

 down, or adnate to the stem, this part being called by some authors the concres- 

 cence ; in fact, the free portion regarded by some as the true leaf, forms only a very 

 small fraction of the leaf substance, and is sometimes designated " leaf scale." 

 The leaf, however, as understood by us, includes the whole of the decurrent or 

 concrescent portion as well as the free end, the former certainly, as that has a 

 true leaf origin and contains as well the essential organs of a true leaf, such as a 

 vascular bundle, the transpiration and assimilating surfaces, chorophyll cells, oil 

 glands, &c. ; the free-end portion is certainly wanting in some of the most essential 

 of these organs that go to make true leaf structure (vide numerous figures given 

 under the species to illustrate these remarks), and so cannot be classed as a leaf. 



Under certain, or most favourable, conditions the three concrescences of 

 the whorl coalesce into one whole, forming as it were a kind of pyramidal com- 

 pound leaf, and almost a perfect triangle in section, just as in some instances of 

 the genus Pinus, i.e., P. cembra, P. Donnel-Smithii , &c., as shown by Masters, 



