20 H. G. SMITH. 
clusions based on evidence of this nature, but rather that. 
further study will demonstrate more completely the indi- 
cations of divergence so obtained. 
Whatever may be the object of the plant in the formation 
of these particular essential oil constituents, it seems 
evident that definite lines of molecular arrangement are 
followed, and it is possible that the formation of these 
characteristic constituents is a part of the economy of the 
plant, leading to the completion of the metabolic processes 
of the particular species in which they occur. It thus 
appears that the influences directing the formation of these 
chemical products are practically constant,.even under 
diverse conditions of natural growth, so that the products 
thus formed have a discriminative value and bear a constant 
relation to other structural characteristics of the plant. 
This chemical feature thus becomes really of morphological 
value, as much perhaps as either a fruit or a bud, although 
the identification is more subtle, and it is of course, less. 
easily available for observation. 
It would be well if the chemical characters of the closely 
related genera of Africa, Tetraclinis of the north, and 
Widdringtonia of the south, were determined so that the 
origin and method of distribution of these, together with 
Callitris, might be more satisfactorily followed. 
Dr. Henry has shown that the sandarac resins of Callitris 
and of Tetraclinis are similar, but more complete investi- 
gation with the exudations of all the species of the three 
genera would probably show a gradation in the percentage 
amounts of the chief resin acids or resinoids, the arrange- 
ment of which in proper sequence—together with the 
determination of their leaf oil constituents—would probably - 
assist greatly their classification. The leaf oils of Tetra- 
clinis and of Widdringtonia have not so far been chemically 
determined, so that comparison in this direction with those 
