The American oaks / by William Trelease.

  • Trelease, William, 1857-1945.
Date:
1924
Catalogue details

Licence: In copyright

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

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    interlock with it, thus giving added protection during the first season to the dormant pollen tubes which apparently use the stylopodium as a hibemaculum. The later lengthening of the ovary and the intrusion into its cavity of three two-ovuled placentae which are at first parietal, render it essentially three celled with an axile placenta at one stage, though its further enlarge¬ ment as the usually solitary seed develops more or less completely obliterates this structure and leaves it distinctly one celled at maturity. Oersted had laid great stress on the importance of the stigma in the classification of oaks, though it affords a character common to groups of species rather than peculiar to species them- Fig. 3.—White Oak Gynaecim (Q. macrocarpa X5). Fig. 4.—Red Oak Gynaecium (Q. imbricaria X25). Fig. 5.—Ovary and Cu- pule (Q. macrocarpa X5). selves. Since the stigmas persist on the fruit, they often afford a dependable indication of the group to which a specimen with partly developed acorns belongs, when other characters may be less convincing. As it is limited now, the genus Quercus differs on this continent from other Fagaceae in a spatulate dilatation cf the stigmas, this enlargement being broad, abrupt, and spreading in the white and intermediate oaks, and gradual on the longer outcurving styles of black oaks. FRUIT. Notwithstanding the great diversity of oak fruits, the most striking fact about them is^that, constantly for each species, they mature in the course of the first season in all of the white oaks, so far as known for America, while in by far the larger number of cases they require two years for development in the black oaks. As has been shown by Hofmeister as early as 1858, and later by Conrad, the biennial-fruited species await fertilization for an entire year after being pollinated, so that the actual period of fruit maturation is the same in all cases. It is so simple a matter to observe whether a tree in autumn bears undeveloped pistillate flowers on the present season’s growth and mature fruit on that of the preceding year, or ripe fruit only and this on the growth of the year, that this character is usually ascertained with certainty; and yet many herbarium specimens of tropical oaks are questionable in this respect for several reasons, and it is quite possible that some of the closely related black oaks that have been dif¬ ferentiated principally on apparently annual or biennial fruit ripening are not really separable even in this respect. Cup.—A special feature of the oak fruit, which experience shows to afford one of the most constant and dependable characters for these plants, is the cup that surrounds the base of the acorn. In the American species it is always covered by distinguishable, if sometimes modified, scales—sometimes, as in Q. lyrata, etc., fused at base. The foliar nature of these has been argued from their occasional teratological replacement by leaves. Formed with the pistillate flower, the cup grows with greater rapidity for a time, so that the partly developed acorn is inclosed in it except for its tip. This condition persists to maturity in Q. lyrata; but it is not to be considered characteristic for a number of species in which, because