26 



developing forms of vestiture such as pilosisin, wax, &c., or creating an essential 

 oil in the leaf texture, so in this section of Conifers a concrescence of the leaf gives 

 the desired security, and consequently these trees can, and do exist in the arid 

 interior of this Continent, where other trees not so provided by nature, might 

 perhaps die. {Vide remarks under C. glauca re movement of leaves of Piiiits 

 ha/apcnsis under climatic influences, and also remarks to account for the 

 decurrence of Callitn's leaves). 



Such a security is not by any means novel, for, as stated above, Lepido- 

 dendron Hickii of the Carboniferous period shows similar furrows in w^hich stomata 

 are placed as in our CciHitris. Does this feature point to a similar climate in those 



bygone ages as that existing with our Callitris 

 \ to-day, viz., that of a comparatively arid nature? 



The free portion of the leaf would appear to 

 varv in length in proportion to its exposure to 

 light, for it is found that the long, pyramidal 

 leaves occur only in the lower, shaded branches, 

 or on trees overshadow'ed by larger ones ; 

 the small, appressed, free portion of the leaf 

 occurring wherever the branchlets are exposed 

 to the full light of day. 



Venation as understood in phanerogams is 

 practicall}^ wanting in the leaves of the Callitris, 

 there being an entire absence of surface veins 

 such as is found in the usual lamina or blade of 

 an ordinarv leaf. The mid-rib is indicated by 

 the very small vascular bundle at the base of 

 the concrescence portion, and midway between 

 the two lower concave surfaces. The leaves of 

 Callitris may, therefore, be regarded as homomorphic, the apparent dimorphism 

 being due to a long or short attachment to the stem, or perhaps, more correctly 

 in this case, the stele, and, as stated previously, primordial leaves proportionately, 

 have just as small a decurrent portion as the normal leaves have a free end. 



Morphologically the primordial leaves may be described as pyramidal, and 

 in section triangular throughout, although in the case of the concrescent leaf it 

 is really only the free end that retains that form. 



As regards adnation applying to the normal form of leaf, ('. Maclcayaiui 

 in some instances forms an exception to the rule, as free, pyramidal leaves 

 obtain almost throughout the whole life of a tree in some cases, so that it is 

 perhaps hardly correct to designate these leaves as ])rjnoiiiial, in fact, it was 



Figure 3. — \n i-arlybranchlt-t showing gradation 

 from int- to decurrent leaves of a Callitris. 

 Nat. size 



