30 PERSISTENCY OF THE LEAVES. 



eoncrescent with it ; on the latest growths both pairs are generally 

 similar and equal. 



Mention may here be made of leaf-like structures that occur in 

 two well-defined genera, and which are popularly called leaves but 

 are not true foliage leaves in the scientific acceptation of the term, 

 although they perform all the functions of such. These are the 

 " needles " of Sciadopitys and the cladodes of Phyllocladus. The 

 " needles " of Sciadopitys present greater resemblance to axial 

 structures than to foliage leaves ; they occupy the axils of leaves 

 of the first order scale-like bodies of deltoid shape which soon fall 

 off, and thus correspond in position with the fascicles of leaves in 

 Pinus but are not necessarily of the same morphological significance.* 

 From the evidence derived from their anatomy and from other 

 circumstances they are more properly regarded as phylloid shoots than 

 as true leaves. 



In Phyllocladus, a genus of Taxads occurring in New Zealand, 

 Tasmania and Borneo, the primordial leaves are small linear organs 

 that usually disappear before th3 end of the third year, and the 

 adult leaves are minute scale-like bodies produced at the base of the 

 shoots in the early spring but soon fall off.t The functions of 

 foliation in this genus are performed by flattened, fan-shaped, leathery 

 expansions of the branchlets termed phyllodes; these phyllodes are 

 arranged laterally on each side of the axis that produces them, much 

 in the same manner as true leaves. 



The persistency of the adult leaves of Taxads and Conifers varies 

 from a single season to several years. In Ginkgo, Larix, Laricopsis, 

 and Taxodium the leaves are deciduous ; in Araucaria imbricata their 

 persistency is the longest yet observed in this country, in many 

 instances from fifteen to eighteen years.J The Kauri Pine of New 

 Zealand, Agathis austrahs, probably retains its leaves under certain 

 circumstances for a still longer period. The greatest variability in a 

 single genus occurs in Pinus ; in P. halepensis, P. Strobus and a 

 few others they fall off at the end of the second year, whilst in P. 

 Balfoiiriana and other species which have their home in the dry 

 climate of the western plateau region of North America they are 

 persistent from ten to fifteen years. In Abies and Picea it is not 

 unusual to find green leaves from seven to nine years old, but the 

 average persistency throughout these genera is from five to seven years. 

 In the Cupressinre the leaves continue green from three to five 



* Masters in Journal of Botany, XXII. (1884), 97, and Journal of the Liimean Society, 

 XXVII. 276, where the subject is fully discussed. 



t In Phyllocladus glauca they are similar to the primordial leaves but rather broader. 

 Kirk, Forest Flora of New Zealand, sub. t. 99. 



J In Araucaria imbricata the length of time during which the leaves remain green 

 on the branches is modified by situation and environment. 



