Fig. 2

Differential interference contrast (DIC) images of Beta vulgaris ovules cleared by the standard procedure with methyl salicylate (a), and the exemplary effects of its modifications tested (b–i). a Ovule with hardly visible embryo sac (dotted line). b Micropylar pole of the ovule with a mature embryo sac after the standard procedure with prolonged incubation time in pure methyl salicylate for up to 4 weeks. c Ovule with a mature embryo sac subjected to shaking and vacuuming during infiltration steps. d Needle-disrupted ovule (arrowheads) with a mature embryo sac. e Over-macerated ovule with constricted embryo sac after pre-treatment with 6% hydrogen peroxide. f Well cleared ovule after pre-treatment with 3% sulfuric acid. Image quality distorted by light reflection. g–i Well cleared ovules after pre-treatment with 0.1 M hydrochloric acid combined with Schiff’s reagent. g Ovule after meiosis with a functional megaspore formed. h Mature embryo sac with visible polar nucleus of the central cell. i Mature embryo sac. ch, chalazal pole of the ovule; dm, degenerated megaspore; es, embryo sac; f, funiculus; fm, functional megaspore; ii, inner integument; m, micropylar pole of the ovule; n, nucellus; oi, outer integument; pc, parietal cells; pn, polar nucleus of the central cell. Scale bars: 20 µm (b–i), 50 µm (a)