Stahl thought that because some parts of plants, like raw cellulose and starch are hard to digest in uncooked form, they would likely not be a part of the hominid diet before fire could be controlled. These parts include stems, mature leaves, enlarged roots, and tubers. Instead, the diet was made up of the parts of the plants that were made of simpler sugars and carbohydrates such as seeds, flowers, and fleshy fruits. Another problem was that some seeds and carbohydrate sources are poisonous. Cyanogenic glycosides, which are in linseed, cassava, and manioc, amongst others, are made non-poisonous through cooking. The teeth of "H. erectus" and the wear on the teeth reflect the consumption of foods such as tough meats and crisp root vegetables.