18 GENUS PESrUS 



THE BARK. 



Bark is the outer part of the cortex that has perished, having been cut oflf from nourishment by 

 the thin hard plates of the bark-scales. In the late and early bark-formation is found a general but 

 by no means an exact distinction between Soft and Hard Pines. In the Soft Pines the cortex re- 

 mains alive for many years, adjusting itself by growth to the increasing thickness of the wood. The 

 trunks of young trees remain smooth and without rifts. In the Hard Pines the bark-formation be- 

 gins early and the trunks of young trees are covered with a scaly or rifted bark. The smooth upper 

 trunk of older trees is invariable in Soft Pines, but in Hard Pines there are several exceptions to 

 early bark-formation. These exceptions are easily recognized in the field, and the character is of 

 decisive specific importance (glabra, halepensis, etc.). 



Among species with early bark-formation are two forms of bark: 1, cumulative, suflBciently per- 

 sistent to acquire thickness and the familiar dark gray and fuscous-brown shades of bark long ex- 

 posed to the weather ; 2, deciduous, constantly falling away in thin scales and exposing fresh red 

 \ inner surfaces. The latter are commonly known as Red Pines, as distinguished from Black Pines 

 with dark cumulative bark. Deciduous bark changes after some years to cumulative bark, and the 

 upper trunk only of mature trees is red. Red Pines, although usually recognizable by their bark, are 

 by no means constant in this character. Oecological or pathological influences may check the fall 

 of the bark-scales, and then the distinction between the upper and lower parts of the trunk becomes 

 lost. 



SUMMARY 



The various characters that have been considered in the previous pages may be classified under 

 dififerent heads, some of them applicable to the whole genus, others to larger or smaller groups of 

 species. 



GENERIC CHARACTERS 



Several characters, quite distinct from those of other genera, are common to all the species. 



1. The primary leaf appearing as a scale or bract throughout the life of the tree. 



2. The bud its constant position at the nodes. 



3. The internode its three distinct divisions. 



4. The secondary leaves in cylindrical fascicles with a basal sheath. 



5. The pistillate flower its constant nodal position and its verticillate clusters. 



6. The staminate flower its constant basal position on the internode and its compact clus- 



ters. 



7. The cone its clearly defined annual growths. 



Pinus is also peculiar in the dimorphism of shoots and leaves and in their constant interrelations 

 with the diclinous flowers. Evolutionary processes develop features peculiar to Pinus alone (the 

 oblique cone, etc.), but confined to a limited number of species. 



SECTIONAL CHARACTERS 



There are several characters that actually or potentially divide the genus into two distinct sections, 

 popularly known as Soft and Hard Pines. 



1. The fibro- vascular bundle of the leaf, single or double. 



2. The base of the bract subtending the leaf-fascicle, non-decurrent or decurrent. 



3. The phyllotaxis of the cone, simple or complex. 



4. The flower-bud, its less or greater development. 



