given plant scaled with the power of the plant's mass during the plant's growth, but a 2001 paper that included various types of unicellular photosynthetic organisms found scaling exponents intermediate between 0.75 and 1.00. A 2006 paper in Nature argued that the exponent of mass is close to 1 for plant seedlings, but that variation between species, phyla, and growth conditions overwhelm any "Kleiber's law"-like effects. Intra-organismal results Because cell protoplasm appears to have constant density across a range of organism masses, a consequence of Kleiber's law is that, in larger species, less energy is available to each cell volume. Cells appear to cope with this difficulty via choosing one of the following two strategies: smaller cells or a slower cellular metabolic rate. Neurons and adipocytes exhibit the former; every other type of cell, the latter. As a result, different organs exhibit different allometric scalings (see table). {| class="wikitable sortable" |+ Allometric scalings for BMR-vs.-mass in human tissue !Organ !! Scaling Exponent |- |Brain || 0.7 |- |Kidney || 0.85 |- |Liver || 0.87 |- |Heart || 0.98 |- |Muscle || 1.0 |- |Skeleton || 1.1 |} See also Allometric law Evolutionary physiology Metabolic theory of ecology Scaling law Rate-of-living theory References Further reading Power laws Ecological theories