Anatomy of the secreting spine (EFN) of <i>Opuntia robusta</i>.

<p>(A) Whole spine showing three structural regions (asc, mer, bme); (B) Basal meristem (bme) and areolar tissue beneath the spine base: black arrows indicate crystals; (C) Middle elongation region (mer); (D) apical secreting cone (asc); (E) Trichomes surrounding the EFN; (F) Xylem (xy) and phloem (phl) located beneath the spine base; (G) Aniline blue and epifluorescence of phloem: red arrows indicate fluorescence of callose at the plate of sieve elements (filter excitation BP 365 nm and emission LP 397 nm); in—internal tissue, ep—epidermis, tr—trichomes,. (A), (B), (C)–bar = 100 μm; (D) (E), (F), (G)–bar = 50 μm.</p

Ultrastructure of extrafloral nectary spine.

<p>(A) Basal cell; (B) Detail of a basal cell showing active endoplasmic reticulum and mitochondria; (C) Middle elongated cells showing some vacuoles and numerous pits (arrows); (D) Detail of middle cells showing abundant Golgi apparatus forming vesicles; (E) Internal cell close to the apical secreting cone showing abundant vesicles and mitochondria; (F) Sub epidermal cell, showing vesicle transport between cells by exocytosis and endocytosis; (G) Epidermal cell below the apical secreting cone; with large vacuoles and numerous pits; (H) Epidermal cell of apical secreting cone (nectar reservoirs) showing cuticle and conspicuous pits (arrows), and external granulose material (nectar). cu—cuticle; cw—cell wall; ex—exocytosis; en—endocytosis; ga—Golgi apparatus; er—endoplasmic reticulum; m—mitochondria; va—vacuole; arrows indicate pits at the cell wall. (A) and (E)–bar = 2.0 μm; (B) and (D)–bar = 0.5 μm; (C) and (G)–bar = 5.0 μm; (H) bar = 1 μm.</p

The effect of sprout length, width, and thickness, air humidity and temperature; on ant number on sprout, during 1 min observation.

<p>The effect of sprout length, width, and thickness, air humidity and temperature; on ant number on sprout, during 1 min observation.</p

Lignification of the apical cone.

<p>(A) Transversal resin section (1 μm) of the apical region (ap) (B) Transversal section of the subapical (sap) region; (C) Transversal region of the basal region of the cone (bac). Observe the thick cell walls of the apex (ap) and some central cells of the subapical region. (A), (B) and (C) bar = 20 μm; (D) bar = 100 μm.</p

Areole of young cladode of Opuntia robusta.

<p>(A) Young areola with extrafloral nectary (efn, arrow) behind a persistent leaf, showing a drop of nectar (close up at the corner); (B) Ant collecting nectar from the extrafloral nectary; (C) and (D) Extrafloral nectary is a modified spine with an apical secretory cone (asc); (E) Distribution of the areola components and diagram. asc–apical secretory cone; efn: extrafloral nectary; lf—leaf; s—lateral spines; sp—central spine; tr—trichomes. (A) and (B)–bar = 2 mm, (C) and (D)–bar = 1mm, (E)–bar = 3 mm.</p

Scanning Electron Micrograph of extrafloral nectary spine.

<p>(A) Whole modified spine; (B) Transition region between middle and apical secreting cone showing intercellular spaces and a broken cell; (C) Apical secreting cone; (D) Detail of the apex. asc—apical secreting cone; efn—extrafloral nectary; tr—trichome. (A)—(C)—hermaphrodite, (D)–feminine.</p

The effect of (A)–(C) sprout size (length, width, thickness), (D) time of observation, (E) relative humidity, and (F) temperature, on per sprout average ants’ number.

<p>Ants were less frequent on wider sprouts (B), and more frequent in the afternoon, when humidity was higher ((E); at the edge of significance: <i>P</i> = 0.065). We present the probabilities obtained in Poisson fixed-effect multiple regression model (PMRM). Regression equation for the significant relationship is depicted with continuous line, and its respective probability of contribution to the PMRM, with bold.</p