14 GENUS PmUS 



ing into smaller tapering strands whose delicate tips converge toward the umbo. Fig. 70 represents 

 a magnified cross-section of half the cone-scale of P. Greggii; at (a) is a compact dorsal plate of bast 

 cells; at (c) is a ventral plate of the same tissue but of less amount; at (b) is the softer brown tissue 

 enclosing the wood-strands (d, d) (the last much more magnified in fig. 69) and the resin-ducts (e, e). 



WOOD STRANDS. 



The wood-strands, forming the axis of the cone, dififer in tenacity in the two sections of the genus. 

 Those of the Soft Pines are easily pulled apart by the fingers, those of the Hard Pines are tougher in 

 various degrees and cannot be torn apart without the aid of a tool. This difference is correlated with 

 differences in other tissues, all of them combining in a gradual change from a cone of soft yielding 

 texture to one of great hardness and durability. 



If a cone scale of P. ayacahuite is stripped of its brown and bast tissues (fig. 66) and is immersed in 

 water and subsequently dried, there is at first a flexion toward the cone-axis (fig. 67) and then away 

 from it (fig. 68). The wood-strands are hygroscopic and cooperate with the bast tissues in opening 

 and closing the cone. This appears to be true of all species excepting the three species of the Cem- 

 brae, whose strands are so small and weak that they are not obviously affected by hygrometric 

 changes. 



BAST TISSUE. 



With the exception of the three species of the Cembrae the inner part of the cone-scales is pro- 

 tected by sclerenchymatous cells forming hard dorsal and ventral plates (fig. 70, a, c). In Soft 

 Pines these cells are subordinate to the more numerous parenchymatous cells, but in Hard Pines 

 the sclerenchyma increases in amount until, among the serotinous species, it is the predominating 

 tissue of the cone-scale, giving to these cones their remarkable strength and durability. 



This bast tissue is hygroscopic and, with its greater thickness on the dorsal surface, there is a much 

 greater strain on that side of the scale, tending to force the scales apart when they are ripe and dry, 

 and subsequently closing and opening the cone on rainy and sunny days. 



The cone, during the second season's growth, is completely closed, its scales adhering together 

 with more or less tenacity. In most species the hygroscopic energy of the scales is suflScient to open 

 the cone under the dry condition of its maturity, but with several species the adhesion is so persistent 

 that some of the cones remain closed for many years. These are the peculiar serotinous cones of the 

 genus. 



THE SEROTINOUS CONE. 



As an illustration of the area to which the adhesion is confined, a section may be sawed from a cone 

 of P. attenuata (fig. 71) . The axis and the scales that have been severed from their apophyses (b) can 

 be easily pushed out of the annulus (a), which is composed wholly of apophyses so firmly adherent 

 that they will successfully resist a strong effort to break them apart. When immersed in boiling 

 water, however, the ring falls to pieces. An examination of these pieces discovers adhesion only on 

 a narrow ventral border under the apophysis and on a corresponding dorsal border back of the 

 apophysis. The rest of the scale is not adherent, so that the seed is free to fall at the opening of the 

 cone. 



The serotinous cone is a gradual development, wanting in most species, rare in a few, less or more 

 frequent in others. A similar evolution of the persistent cone, of the oblique cone and of the cone- 

 tissues has been already discussed. All these progressive characters culminate in mutual associa- 

 tion in P. radiata and its allies. The result is a highly specialized fruit that should convey taxonomic 

 significance of some kind. 



With all serotinous species that I have seen, some of the trees open their cones at maturity, others 

 at indefinite intervals. That is to say, the seed of a prolific year is not at the mercy of a single, per- 



