Of these new characters, the thick wing-blade attains such proportions in the three species of the Macrocarpae that they can be grouped apart. But the characters that finally culminate in a lateral oblique serotinous cone are so gradually and irregularly developed that they offer no divisional distinctions. With the aid of wood and leaf characters, however, groups can be established which preserve the evolutionary sequence and, at the same time, the obvious affinity of the species.
| Wing-blade thin or slightly thickened at the base. | ||
| Cones dehiscent at maturity. | ||
| Pits of the ray-cells large | X.Lariciones | |
| Pits of the ray-cells small | XI.Australes | |
| Cones serotinous, pits of the ray-cells small | XII.Insignes | |
| Wing-blade very thick | XIII. Macrocarpae | |
The species of this subsection are very difficult, if not impossible, to classify by the usual method, which groups all species under a few characters assumed to be invariable and of fundamental importance. Such a method can be successfully applied to the Soft Pines and to some of the Hard Pines, but cannot be applied to all the Hard Pines without forcing some of them into unnatural associations.
To take an example, the group Pseudostrobus, characterized by pentamerous leaf-fascicles, appears in many systems. In this group are placed P. Torreyana and P. leiophylla. Another group, with trimerous fascicles, contains P. Sabiniana and P. taeda. Now there are no two species more obviously related by important peculiarities than P. Torreyana and P. Sabiniana; nevertheless they are, by this method, kept apart and associated with species which they resemble in no important particular.
An attempt is made here to avoid such incongruities. Groups X, XI and XII represent different stages of evolution. In the Lariciones the cone is symmetrical, and dehiscent and deciduous at maturity, while the spring-shoot is uninodal. In the Australes there is a similar cone, but the spring-shoot gradually becomes multinodal. In the Insignes the cone is oblique, persistent and serotinous, and the spring-shoot is multinodal.
These definitions state the degree of evolution attained by each group, but not all the species of a group conform exactly with its definition. In each group are species with a characteristic of another group. Among the Lariciones are a few species with both symmetrical and oblique cones, and two with persistent cones. Similar exceptions occur among the Australes. Among the Insignes are a few species with symmetrical cones, and two with cones that are rarely, if ever, serotinous.
There is, however, no difficulty in fixing the systematic position of these exceptional species through other characters which show their true affinity. They are placed with the species which they most resemble. Their exceptional characters are merely the evidence of the evolution that pervades and unites the groups. Therefore the definition of a group is not necessarily the exact definition of its species, and a species is placed in a group because all its characters, specific and evolutional, show a closer affinity with that group than with the species of any other.
X. LARICIONES
Pits of the ray-cells large. Cells of the leaf-hypoderm uniform. Spring-shoots uninodal. Cones dehiscent at maturity.