The cuticle of the thistle plant controls its degree of elasticity by regulating the degree of hydration of constituent trihydroxy fatty acids compounds.

The external coating, or cuticle, of plant surfaces protects from desiccation and predation, but also add to plant structural support as plants age. Younger plants, and newer growth on older plants, have a greater degree of elasticity, perhaps because they do not need to support a large mass or because they need greater hydration during growth. Initially, the OH (hydroxyl) groups of cuticle-based trihydroxy fatty acids function to increase tissue hydration. As tissues age, cross-linking between hydroxyl groups increases, making them less available for hydration leading to greater tissue rigidity.

References

"The cuticle is a complex structure of soluble lipids, lipid polymers and polysaccharides. In addition to its functions to reduce water loss and provide a protective barrier, its mechanical properties may be significant to plant growth and development...We discovered that the ratio of carbohydrates to lipids is 1:7 in floral parts but 2:1 in leaf cuticle. Esterified cutin components represented about 80% of the cuticle and di-hydroxyhexadecanoic acids were the major monomers of cutin, regardless of origin. The cutin of elastic tissues is characterized by a higher content of tri-hydroxy monomers than the cutin of rigid tissues. The data suggest that hydroxyl groups enhance the hydrophilic character of the cuticle and contribute to cuticular elasticity." (Marga et al. 2001:841)

"The most remarkable difference between elastic and rigid tissues was the 4- to 8-times-higher content of trihydroxy fatty acids of the elastic tissues. Because a similar decrease in the relative amount of the trihydroxy C18 monomers occurred during cuticle development in ripening tomatoes, elasticity appears to be associated with a higher number of hydroxyl groups. The higher amount of trihydroxy monomers in the cuticle of elastic tissue argues for increased hydration of such cuticle and against the potential for increased cross-linking capability, which would reduce flexibility." (Marga et al. 2001:846)

"[T]he elasticity of cuticle depends on the amount of hydroxylated fatty acids. Contrary to the notion that more hydroxyl groups can serve as potential sites for cross-linking, the OH groups apparently determine the hydrophilicity of the cuticle and facilitate impregnation with water. As the cuticle ages, natural dehydration, possibly in conjunction with the increase of cross-linkers, most notably phenolic compounds, may lead to the often described hardening of mature cuticle...The content of phenolics was correlated with cuticular rigidity. Similar to the strengthening of the cell wall by lignins, the cuticle can also be expected to gain mechanical strength by increased cross-linking of phenolic compounds. Cutin-bound phenolics decreased as fl?oral tissue became more elastic." (Marga et al. 2001:847)

"Hydration of the membranes caused a decrease in the modulus of ∼35–50% and generally increased the total extensibility. These results indicate that the mechanical properties of the biopolymer are considerably influenced by hydration and, as a consequence, water acts as a plasticiser, increasing the extensibility at a given stress as well as the viscous component." (Bargel et al. 2006: 902)

Journal article
Biochemical analysis of elastic and rigid cuticles of Cirsium horridulumPlantaNovember 6, 2009
Françoise Marga, Thomas C. Pesacreta, Karl H. Hasenstein

Journal article
Evans Review No. 3 : Structure–function relationships of the plant cuticle and cuticular waxes — a smart material?Functional Plant BiologyFebruary 10, 2006
Hendrik Bargel, Kerstin Koch, Zdenek Cerman, Christoph Neinhuis

Organism
Terrible ThistleCirsium horridulumSpecies