For Acorns, Size Matters—and So Does Shape

The fruits of Quercus are a source of energy that serves to generate new individuals, thanks to the energy content stored in the cotyledons. The cotyledons join forces with the embryo to generate new individuals, first developing a long taproot and then an epicotyl with small leaflets, followed by primary leaves, and subsequently growing in a process that differs according to the characteristics of the environment and the genetics of each species. The content of the seed has double protection: a thin seed coat or tegmen, the remnants of the inner integument or membrane covering the original fertilized ovule, and a thick, hard shell (external cover or testa) that develops from the outer integument covering the gynoecium, which contains the four ovules that typically make up the female flowers in the genus Quercus. The acorn is crowned by a trifid apex, the remains of the stigma of the original female flower, and is connected to the tree by means of an inferior cover and peduncle, known as cupule or acorn cup, covered in small to minuscule bracteoles, which are arranged serially and are of varying consistency and morphology. The cupule can be compared to the calyx or inferior whorl in the flowers of other plants such as roses, jasmines, and wallflowers.

This fruit structure, which can be simple or complex, is variable in Quercus species. Even within a single species there are variations in morphology, consistency, and appearance of the acorns depending on numerous factors, both environmental and genetic. In this large and complex genus, different taxa (species, subspecies, varieties, and forms) have been named based on the variations in acorn shape. The aim of this brief essay is to set out the more frequently encountered morphological variations in the genus Quercus and their possible causes and/or consequences within the niche or habitat occupied by the individual plants.

Size

The characteristic that most often varies in acorns is the size. Some species have large acorns, other have small ones, and others medium sized. The reason that acorns come in different size is not clear in all cases, as acorn size seems to be related to environmental conditions or habitat, to differing reproductive cycles, and to genetic causes.

In the case of large acorns, some weighing over 35 g and found in species such as Q. crispula Blume, Q. gambelii Nutt., Q. macrocarpa Michx., or Q. rotundifolia Lam., they originate as a consequence of the presence of polyembryonic or polyspermic fruit, in other words, acorns that contain more than one embryo and hence more than two cotyledons and more than one seed coat. These in turn result from the fertilization and posterior development of more than one of the four ovules contained in the female flower. This situation leads to large acorns and the emergence of more than one seedling from a single acorn. This type of acorn can be the consequence of the fertilization of more than one ovule (polyspermy), or of the duplication of an embryo once a single ovule has been fertilized (polyembryony). In the first case the embryos are genetically different (different ovules, different grains of pollen), while in the second case the embryos are genetically identical (same ovule, same pollen), though with a high probability of spontaneous mutation. In all these cases, acorn size has a genetic component.

In some species, such as Q. suber L., which usually has two reproductive cycles in the same year, there are two acorn crops in autumn-winter. There is a first crop of acorns proceeding from female flowers produced in the phenological phase (flowering period) of late summer to early autumn of the previous year, that mature in October/November of the following year; and then a second crop of acorns proceeding from female flowers emerging in spring of the current year, that mature in the months between November and January. The acorns from the first crop are two to three times larger than those of the second crop, principally because they have an extra three to four months in which to develop than second crop acorns. In this case, acorns vary in size within the same individual tree and species due to differences in reproductive cycle.

Variations in acorn size due to environmental causes are often fixed genetically. Three kinds of research have been carried out: a) acorns of the same species and different latitude (Q. macrocarpa); b) acorns of different species and different longitude (oaks from the Atlantic coast of the United States and from the Californian coast); and c) acorns from the same species and different altitude (Q. hypoleucoides A. Camus). In all cases the results have been inconclusive and offer several possible options to explain the variations in acorn size: a) causes related to climatic conditions (species growing in dry areas tend to have smaller acorns); b) causes related to latitude (more northerly latitudes have smaller acorns); c) causes related to dispersion (small-acorn species disperse better and are colonizing species). In any case, there is a clear connection between the volume of the cotyledons and the biomass produced by the radicle, so that larger acorns produce larger or longer radicles, which explains the large acorns of Q. insignis M. Martens & Galeotti, Q. lamellosa Sm., or Q. macrocarpa, which require deep-reaching radicles in order to become established in the habitats (specifically, the type of soil) they occupy.

Small-acorn species, or small-acorn varieties of some species, could be explained in various ways: a) ease of dispersal by birds; b) the possibility of having a larger number of acorns per tree, which increases the number of progeny and colonization of habitat; and c) defense against attack by Curculionidae (weevils), as a larger number of acorns increases the chance that some acorns will escape attack and remain viable.

Shape

Acorn shape varies, and in fact not all species have acorns that separate from the cupule or acorn cup. For example, a large number of species in the subgenus Cyclobalanopsis germinate while still inside the cupule. It is important to take this characteristic into account, because the effective shape of the acorn depends on whether it separates or not from the cupule.

The shape of acorns (not taking into account the cupule) varies from globose or spherical to fusiform (tapering at each end), the latter varying in length. These two large groups, globose and fusiform, appear independently in each species, so that we find species with globose acorns and others whose acorns are exclusively fusiform.  This characteristic is significant, because a spherical object is better adapted to rolling along a surface than a fusiform one. Also, a round acorn can be grabbed more easily by small paws than one that is fusiform, which by contrast has a shape that adapts well to the beak of a bird. So the acorn shape may correspond to a preferred mode of dispersal, whereby spherical acorns, without excluding ornithochory (dispersal of seeds by birds), may be better adapted for physical dispersal (water, wind, slope) and for dispersal by mammals, while fusiform acorns, which may also be dispersed by physical means and by mammals, may be better adapted for bird-dispersal.

Examples of oaks named according to the shape of their acorns include Q. fusiformis Small., Q. avellaniformis* [literally: hazelnut-shaped] Colmeiro & E. Boutelou (a varietal form of Q. rotundifolia Lam., now known as Q. rotundifolia var. avellaniformis* (Colmeiro & E. Boutelou) F.M. Vázquez, S. Ramos & S. García) and Q. globularis* Djav.-Khoie (a synonym of Q. ungeri* Kotschy, which is in turn considered by some sources to be a synonym of Q. ithaburensis subsp. macrolepis (Kotschy) Hedge & Yalt.).

*These names have not been accepted and their status is under investigation

Translated by Roderick Cameron