Hosting ants confers effective protection against herbivores, and swollen, hollow stems functioning as domatia have evolved many times in plants. Blatrix et al. study the allometry of stem secondary growth in Leonardoxa africana subsp. africana, whose internodes display both a solid basal segment and a hollow distal part (the domatium), thus allowing paired comparison of investment in wood. They show that the cost of producing a domatium is very high at the beginning of internode growth, but quickly tends to zero. The total cost of this defence trait is much higher early in ontogeny of the plant, when allocation to growth is most crucial, suggesting that domatia confer great benefits during these life history stages
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