Annonaceae as a family is easily recognized not only by its flowers, fruit and vegetative habit, but also by the characteristic wood. The vessels are diffusely arranged, solitary and in short radial multiples. Perforation plates are simple and the alternate intervessel pits are mostly minute or small. Two features of the xylem structure stand out in particular: first, the broad and high multiseriate rays and, second, the numerous narrow parenchyma bands that are visible in transverse sections as more or less continuous tangential lines, perpendicular to the rays. The resulting cobweb- to ladder-like (or reticulate) appearance in cross-sections, often visible even to the naked eye, is found in all Annonaceae, even in the first-formed xylem (Fig. 40A). Even the deviating genus Tetrameranthus R.E.Fr., with tetramerous flowers and spirally (rather than distichously) placed leaves, shows this striking structure (Ter Welle, 1985).
The same can be said of species growing in moderate climates or of those with a climbing habit. Although taxa belonging to these categories often deviate in certain characters, the general annonaceous structure is always present.
There is general consensus that Annonaceae forms a natural group, and this is supported by the high homogeneity in wood structure. Various authors over the years have arrived at this conclusion: for example, Solereder (1899, 1908), Moll & Janssonius (1906), Hess (1946), Metcalfe & Chalk (1950), Vander Wijk (1950), Ingle & Dadswell (1953), Vander Wijk & Canright (1956), Gottwald (1977), Ter Welle (1984, 1998) and Metcalfe (1987).
In the APG III classification (APG III, 2009; Stevens, 2001 onwards; http://www.mobot.org/MOBOT/research/APweb/; date of access 25 Sep 2011) Annonaceae is placed in the order Magnoliales with Degeneriaceae, Eupomatiaceae, Himantandraceae, Magnoliaceae and Myristicaceae (Sauquet et al., 2003).With the exception of Himantandraceae, these families show scalariform perforation plates (Degeneriaceae, Eupomatiaceae, Magnoliaceae) and/or reticulate to scalariform intervessel pits (Degeneriaceae, Eupomatiaceae, Magnoliaceae, Myristicaceae). Himantandraceae differs, among other characters, in its longer radial rows of vessels, the continuous parenchyma bands and the presence of prismatic crystals. In none of the above-mentioned families, however, is the reticulate parenchyma pattern, characteristic for Annonaceae, found.
A wealth of information is available from North Carolina State University (NCSU; Raleigh, NC, USA) on the website InsideWood (http://insidewood.lib.ncsu.edu/search/). This website includes microscopic data, often illustrated, on the wood anatomy of (currently) > 2500 genera belonging to nearly 300 families, and it is a valuable tool for the identification of wood. From a search of the InsideWood database applying the International Association of Wood Anatomists (IAWA) wood characters, the combination of wide and high rays (characters 98, 102) and seemingly reticulate parenchyma (characters 86, 87, 88) results in six families. Huaceae, Icacinaceae and Loganiaceae are represented by only one species each. The other three families are represented by more species: Annonaceae (14), Lecythidaceae (7) and Malvaceae (5). In the APG III classification, three of these families (Icacinaceae, Lecythidaceae and Loganiaceae) are found in the asterids, two families (Malvaceae and Huaceae) in the rosids, and Annonaceae is the only one placed in the magnoliids.
When six more features are added in the database search based on our description of Annonaceae, referring to vessel distribution, perforation plates and pits of vessels, rays and fibres (IAWA characters 5, 13, 22, 25, 30, 61) and no mismatch is allowed, a search produces seven hits, four of which are for Annonaceae. This low number is caused by the variation found in most wood anatomical features. When one mismatch is allowed, in order to address this problem, the number of hits for Annonaceae increases greatly (43). Twelve other families, all found in the rosids or asterids, are far less often retrieved, Malvaceae being second best with 14 hits. Annonaceae remains the sole representative of the magnoliids.
Given the wood anatomical differences mentioned earlier, it is not surprising that the families considered to be most closely related to Annonaceae were not found when scanning the InsideWood database (only Degeneriaceae is not represented in the database).
Although wood anatomical characters are usually variable at the familial level, the character combination exhibited by Annonaceae is more or less homogeneous. Thus, the suite of wood anatomical characters discussed above renders taxa belonging to this large family easily recognizable and surprisingly distinct from other Magnoliales.
In this article, a general description of the microscopic wood anatomy of the family is given on the basis of data from the literature and personal observations. A few microphotographs are added. For further illustrations of microscopic features, we refer to the website Insidewood (2004 onwards), which includes photographs of transverse and tangential sections of c. 35, mainly Neotropical, genera of the Utrecht Wood collection.
In addition to the family description, we provide a survey of end-grain photographs of 66 genera and > 90 species of Annonaceae, all from material in the Utrecht Wood collection, which at present is housed at the Netherlands Centre for Biodiversity Naturalis, Leiden, the Netherlands.
Hand-lens images have somewhat suffered from neglect in past decades, most papers showing microphotographs only. The usefulness and advantages of hand-lens photographs, especially end-grain photographs, are discussed by Westra & Koek-Noorman (2004) in their recent wood atlas of Euphorbiaceae s.l. Good examples of atlases of wood photographs through a hand lens, covering a large number of families, are found in Pfeiffer (1926), Lindeman & Mennega (1963) and Ilic (1991).
A preliminary survey of mainly end-grain photographs from a selection of Annonaceae was presented in an informal paper by Westra & Koek-Noorman (2003). They illustrated that, in certain cases, despite the homogeneity within the family, differences observed with a good hand lens can be an important help in identification, particularly at the level of genera or below.
The advantages of the use of a hand lens are obvious: ease, speed and the possibility to observe a relatively large area at a single glance, although the number of characters that can be easily observed in end-grain photographs is restricted. The differences are often slight and difficult to summarize in a few words, and illustrations are indispensable.
The variation seen in lens key characters is discussed against the framework of phylogenetic relationships in the family (Richardson et al., 2004; Chatrou et al., 2012). To facilitate the comparison, the photographs are arranged according to the clades in figure 1 of Chatrou et al., 2012. Additional remarks on microscopic features are given when appropriate.