Most plants use photosynthesis to fix carbon dioxide from the atmosphere into sugars. Plants build their rigid cell walls out of cellulose (long polymers of β1-4 linked glucose). Cellulose is difficult to digest, but animals such as ruminants and primates with specially adapted fermenting guts (leaf monkeys and gorillas) can extract substantial energy from cellulose with the help of cellulose-digesting microorganisms. Humans can digest disaccharide sucrose and poly-glucose starch (which, unlike cellulose, consists of glucose in α1-4 glycosidic linkages). See starch. Grasses such as rice, wheat, barley, millet, and corn also store sugars as starch in the endosperm of their seeds. The mere act of chewing starch introduces salivary amylase enzymes, which cleave starch into shorter and sweet-tasting oligosaccharides (malt). There is genetic evidence for human adaptation to the consumption of starch, as human populations with longer histories of grain agriculture have larger copy numbers of functional amylase genes in their genomes. (All humans have higher amylase gene copy numbers than any of the great apes.) Grain seeds themselves contain amylases (hydrolases). Germinating the seeds serves to activate the amylase enzymes; when the crushed seeds are mashed in warm water, sweet-tasting malt is generated. However, the human love for sugars and sweet taste can be a liability, as it favors a taste for one of the major byproducts of sugar fermentation: the ethanol found in alcoholic drinks. See fermentation.