The American oaks / by William Trelease.

  • William Trelease
Date:
1924
Catalogue details

Licence: In copyright

Credit: The American oaks / by William Trelease. Source: Wellcome Collection.

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    [VOL. upward, so that at the middle the petiole contains a single somewhat three-sided curved bundle consisting of phloem on the outside and xylem within, encircling the central mass of medullary parenchyma and sheathed on the outside by a sclerified white pericycle between which and the epidermis lies the rather thick cortical parenchyma. In our common deciduous oaks this vascular ring is reinforced by an inner transverse plate with xylem facing ventrally and phloem dorsally, lying in the medullary parenchyma, a circumstance apparently not connected with the obvious taxonomic differ¬ ences of the species, since the structure is comparable in both white and black oaks; but it has been pointed out by Bosseboeuf that this inner strand of xylem and phloem is characteristically absent from a number of evergreen oaks. Margin.—-One accustomed to the oaks of northern countries, on either continent, naturally thinks of them as having deeply lobed leaves, the lobes rounded and with rounded sinuses in European and many Amer¬ ican species, but in North America occurring quite as often with sharp bristle-tipped segments and sometimes acute sinuses. Some of those of northern Asia are serrate as are some of the temperate American species, and a small number of the latter are entire. A glance through the accompanying plates will show that very few of the tropical American species are deeply lobed, and that many of them are entire. Most oak leaves are toothed in one way or another, and when entire they may be mucronate if not artistate from the midrib or the tips of the primary veins. Veining.—As Oersted has indicated, the veining of the leaves presents several distinct types, especially in meshing and looping; but the differences are not easily expressed in a few words, and they undergo such marked changes between lobed and entire leaves—often of closely related species or of forms of a single polymorphic species—that I have not found it possible to rely on them to any considerable extent. It is to be noted that this is one of the most utilized characters in the description of fossil leaves, and von Ettingshausen has made it the subject of extended comparative studies, illustrated by his unequaled type of nature prints. More practically available than their direction and branching is the elevation or depression of the veins on the surface. A number of groups of the dry Tropics are well marked in having their veins sunken below the upper leaf surface, either wholly—when they are spoken of as reticulately impressed or rugose—-or as to the branches of the midrib when the term pinnately impressed has been applied to them. On the lower surface, and in many species on the upper face as well, the veins are generally prominent. Relatively few species have such thick and firm leaves that the veins are neither impressed nor raised above the surface. To a certain extent the number of principal veins and the angle that they form with the midrib are characteristic, but neither feature has seemed to me to possess sufficient constancy to make its use very depend¬ able in contrasting closely related species. Pubescence.—Though many of the oaks are hairy as they develop from the bud, a very large number of them become nearly or quite glabrous as their several parts mature. This is particularly true of the upper leaf surface, so that the persistence of scattered short stiff hairs on the upper face of the leaves assumes importance for the recognition of such species as Q. stellata (pi. 10), as the velvety coating of the upper surface does for such groups as the Chihuahuenses. The lower surface retains its pubescence much more commonly. The Virentes, Scytophyllae, and Crassifoliae, for instance, not only exemplify this fact, but illustrate extreme types of such hairiness; and the Arizonicae and Furfuraceae present other and different illus¬ trations. Where the pubescence normally falls, as the leaves mature, its disappearance is not always or uniformly complete. Vestiges are to be expected in sheltered places, such as the groove along the midrib above or the sides of the midrib and main veins beneath; and in many species {e. g., Q. palustris, pi. 10) characteristic tufts of brown hairs—the acaro-domatia of Lundstrom—persist in the axils of the veins on the lower surface of the leaf. Such hairs, like all of the oak trichomes except for a few septate glandular hairs, are finally thick walled, not septate, and stellately tufted or sometimes somewhat peltately connate: as Engelmann has pointed out, these are the “golden scales” of the Californian Q. cJirysolepis. Fig. 1.—Deciduous oak petiole (Q. palustris X25).
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