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 timbers, a cause of disease in animals and humans, and one of the greatest causes of agricultural losses.

　　3. 逗号用来分隔与句子其他部分密切相连的简短插入语或旁白。(较长的，更为突兀的或复杂的插入成分的则用破折号或圆括号。)

　　Chemical fertilization,for example, helps to produce better crops, but is harmful to the environment.

　　4. 在并列形容词，即分别修饰同一个名词的形容词之间使用逗号，但也有的形容词之间不加逗号的。

　　For men, heroism was usually described as bravery and the active, successfulovercoming of adversity.

　　5. 用逗号来分隔非限定性修饰语，即该修饰语对于句子的意义并非必不可少。非限定性修饰语与限定性修饰语的区别在于，它即使被省略，也不会改变句子的主要意思。

　　(1) 同位语

　　The canopy, the upper level of the trees in the rain forest, holds a plethora of climbing mammals of moderately large size, which may include monkeys, cats, civets, and porcupines.

　　(2) 从句

　　The canopy, the upper level of the trees in the rain forest, holds a plethora of climbing mammals of moderately large size, which may include monkeys, cats, civets, and porcupines.

　　7. 在以"月、日、年"为次序排列的日期间使用逗号。如果这种次序的日期出现在句子中间，则在"年"后也加上逗号。

　　Martin Luther King, Jr., was born on January 15, 1929, and died on April 4, 1968.

　　但在以"日、月、年"为次序排列的日期间，不使用逗号。

　　Martin Luther King, Jr., was born on 15 January 1929, and died on 4 April 1968.

　　在"月"和"年"之间，或"季节"和"年"之间，不使用逗号。

　　The events of July 1789 are as familiar to the French as those of July 1776 are to Americans.

　　发生在1789年7月的那些事件对于法国人来说，就像美国人对于发生在1776年7月的那些时间一样熟悉。

　　8. 在表示地名时

　　Pandas come from China, Asia.

　　9. 用以将一导言的或转变语气的词或短语(如therefore, however, by the way, for instance)与句中其余部分隔开。

　　On the other hand, fungi bring about the decomposition of dead organic matter, thus enriching the soil and returning carbon dioxide to the atmosphere.

　　10. 书写对话时用于某某说等词语前后。

　　"Come back soon," she said.

　　11. 用于短小的引语前。

　　Disraeli said, "Little things affect little minds."