1. 在并列连词(and, but, for, nor, or, yet )前使用，用来连接句中的各分句。
The principal reason for this is that none of them possesses chlorophyll, andsince they cannot synthesize their own carbohydrates, they obtain their supplies either from the breakdown of dead organic matter or from other living organisms.
2. 用逗号来分隔一系列单词、词组和从句。
Furthermore the walls of fungal cells are not made of cellulose, as those of plants are, but of another complex sugarlike polymer called chitin, the material from which the hard outer skeletons of shrimps, spiders, and insects are made.
They are a major cause of structural damage to building
