Quantitative differences in succulents
This is an image of two different types of Haworthia, a type of Succulent. These are water-retaining plants adapted to more arid climes, and store their water in leaves, stems, and/or roots. It is interesting to me that the taller Haworthia in this picture has thicker stems that are visible above ground, whereas the stems of the smaller Haworthia are at soil level. Perhaps more water is stored in the thick stems of the taller Haworthia, possibly making it a hardier plant than the other. The quantitiative trait, then, would be the height of the thick stem above soil level, therefore indicating more retained water and a wider capacity for survival in harsher conditions.