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The Classification of Conifers. 
a genus as Setiecio, or within families such as Euphoi biaceae and 
Rosaceae; in each of these cases the leaves may be seen to be the 
most plastic organs of the plant; there may be, within undoubtedly 
natural families, both simple and compound, opposite and alternate, 
stipulate and exstipulate leaves, of almost every conceivable shape. 
Where only one type of leaf, or only one “habit” is found in a family, 
it is usually where the geographical range of the family is very 
limited, or is such as to expose the plants to only one type of climate, 
as in the Proteaceae and in several tropical families. Among the 
Gymnosperms, the Gnetales offer a good example of the enormous 
differences in habit which may be found within a somewhat narrow 
circle of affinity. 
To approach the same problem from the opposite direction, w r e 
have the well known constancy of vegetative characters in plants 
admittedly unrelated (or only distantly related), when these are 
grouped in certain habitats which offer very characteristic ecological 
conditions ; as examples the best known are probably the trees of 
tropical rain-forests and the “ bush ” vegetation of the Mediterranean 
region, south-western Africa, and parts of Australia. 
There does not seem any adequate reason for supposing that 
leaf form and arrangement have any greater fixity in the Conifers 
than they have in other plants, yet we still find systematists 
separating two so-called “ tribes,’’ Cupresseae and Taxodeae, 1 almost 
solely on the basis of whether the leaves and the cone scales are 
opposite or alternate. 
While the more recondite characters of the genera concerned 
were largely unknown the leaf characters offered a convenient 
means of cataloguing the species, but now that the internal structure 
and development of the reproductive organs are known in a large 
proportion of cases, it is time that the principles upon which the 
limits of the tribes and families are based should be somewhat less 
seriously behind the present status of botanical progress. 
II. —Characters Important in Classification. 
In theory, though unfortunately not always in practice, it is 
generally conceded that the stability and fixity of plant parts or 
organs is more or less proportional to (a) their distance from the 
surface of the plant, and (b) their proximity to, or connection with, 
the reproductive structures. Applying this theory, we see that the 
least important characters of the plant to the systematist ought to 
1 Throughout this paper the terminations for families, tribes, etc., as 
recommended by the international congress of botanists at Vienna (1905) and 
Brussels (11)10), have been used, whether or not they have been customarily 
so used previously. 
