We use the classification system when grouping organisms, which are very diverse. The sub-group with the least amount of diversity is the species. The measure of diversity in this classification will of the variation in genome and physical characteristics. The species have a slight variation in the genome and phenotype compared to the amount of variation in a genus, where related species are clubbed together. The amount of diversity increases from the nodes of a classification tree to its root or parent (kingdom).
Angiosperms are flowering plants, so I'd say flowers and the parts of flowers are a pretty common characteristic.
Because it transports energy for all cellular metabolic activities.
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