FOREST TREES OF THE PACIFIC SLOPE. 205 



rowed to about three-fourths of an inch, gradually increasing in width, toward 

 and above the middle, to about 1 or 1| inches; then tapering to a stiff sharp 

 point (fig. 81, b). The blade is thin and the edges are strongly curled or rolled 

 from above the middle to the point. The edges are, moreover, conspicuously 

 marked with thread-like fibers, which are frayed from the borders as the leaf 

 grows older. A single branched cluster (about 14 to 16 inches long) of flowers 

 is borne from among the uppermost green leaves. The pulpy, sweetish fruit 

 (fig. 81, c), ripened late in August or early in September, is on a slender droop- 

 ing stem. It is 34 or 4 inches long, blunt at the ends, the top end having a short 

 thick point. The flat seeds, packed in G chambers of the fruit, are black (fig. 

 81, (I). Wood : Nothing can now be said of the wood of this yucca, except that 

 it is lighter and somewhat softer than that of the Joshua tree. 



Longevity. — No definite statement can be made concerning the age limit of 

 this yucca, which, however, can hardly be less long-lived than the Joshua tree. 

 Messrs. C. R. Orcutt and S. B. Parish, who know the tree yuccas from long 

 observation, both inform the writer that the Mohave yucca is an exceedingly 

 persistent but very slow grower in its native habitat, scarcely any change having 

 been perceived in trees under observation for the last twenty-five years. 



RANCK. 



From northeastern Arizona and southern Nevada across the Mohave Desert into Cali- 

 fornia, and from the southern base of the San Bernardino Mountains to the coast rnd 

 northward to Monterey, sometimes ascending mountain slopes to 4,000 feet 



OCCURRENCE. 



Similar to Joshua tree. 



DICOTYLEDONES. 



The trees of the great dicotyledonous class are so called because the germi- 

 nating seeds produce two seed-leaves, or cotyledons. They have broad leaves, 

 with a central vein and a network of smaller connected veins. They are further 

 Characterized by having the non-resinous wood of their trunks in annual layers, 

 which appear as concentric rings on a cross-section of the trunk. Each layer is 

 formed just beneath the living bark and over the layer produced the previous 

 year. This mode of diameter growth gave rise in earlier days to the class name 

 " exogens," or outside growers, in contradistinction to " endogens," or inside 

 growers, a class name then given to the trees we now more generally call " mono- 

 cotyledones." The two terms, " endogens " and " exogens," originated when 

 knowledge of how members of the two classes grow was incomplete. Later 

 studies show that the term " exogens " is still correctly applicable to all dico- 

 tyledonous trees, but that the term " endogens " does not express the manner in 

 which monocotyledones actually make their diameter growth. Monocotyle- 

 dones were once thought to increase in diameter by the addition, each year, of 

 scattered woody fibers at the center or pith of the tree, thus gradually crowding 

 the woody tissue previously formed to the outside of the trunk. In other words, 

 the outside of the trunk was believed to have once occupied the center of the 

 stem. We now know, however, that these trees grow in diameter by laying on 

 tissue outside of that formed the previous year, but not in a distinguishable 

 layer as in dicotyledonous trees. Diameter growth of the gymnosperms i pines, 

 spruces, firs, etc.) is produced in exactly the same way as in dicotyledonous 

 trees, but the oleo-resinous woods of the former distinguish them from the Latter 

 Hass. It is true, indeed, that the wood of some of our broadleaf trees contains 

 resinous matter, but it is not in any high degree oleo-resinous, as In gymno- 



1518S— OS 14 



