1. Introduction1．1 Research BackgroundIn recent years，second language vocabulary acquisition has become an increasingly interesting topic of discussion for researchers，teachers，curriculum designers，theorists and others involved in second language learning．Both learners and researchers see vocabulary as being a very important，if not the most important，element in language learning．Acquisition of vocabulary has assumed a more important role，and as some would argue，the central role in learning a second language. No matter how well a student learns grammar, no matter how successfully the sounds of L2 are mastered，without words to express a wider range of meanings，communication in a L2 just cannot happen in any meaningful way．Therefore，we can safely draw a conclusion that lexicon may be the most important component for learners．However, present lexical teaching and learning do not have a rosy picture．In practice，there are two misconceptions about vocabulary learning．Firstly, some English students follow the traditional pattern of rote retention，ignoring the fact that vocabulary is easier to learn in context than in isolated word lists，and the fact that knowing a word is more than knowing its meaning．In practice，they merely memorize as many words as they can．Students，as a result，put accumulation of English vocabulary above everything in the learning process，Holding such a notion，they adopt whatever methods they believe to be effective to expand their vocabulary size，Some students resort to word lists for rote retention，memorizing the words letter by letter．Some students memorize the words in a dictionary according to the alphabetical order．Others rely on their mother tongue in learning English，putting every English word(even very simple lexical items)，into Chinese and the other way round．Though for a short period they might achieve a rapid vocabulary growth，the words are “easy come easy go” and only stay at the recognition level．Consequently, they can neither be long retained nor correctly used．Moreover，because they hold the wrong idea that learning English is just memorizing the vocabulary，students will not grow to appreciate the beauty of English and the way it conveys the ideas and emotions，As a result，despite the great effort they have spent on memorizing the words，they become more frustrated than ever before，for eventually they come to realize that the English words have nothing to do with thereal life．For example，it is usually the case that a certain student has passed the TOFFL exam with flying colors，but when he goes to the English-speaking country, he is still at a loss what to do，because he couldn’t understand what native-speakers have said，nor can he speak with them.Secondly, a number of students，in spite of being aware of the great role of context and incidental learning in vocabulary acquisition，ignore the importance of another instructional technique：intentional learning．However, in fact，there are several prerequisites for vocabulary acquisition through reading incidentally．For example，one of the prerequisites is that students have to read extensively．But it is usually the case that students are not diligent enough to read all the reading passages in the textbook，let alone the reading materials which are supposed to be finished outside of the classroom.Furthermore，learners’ proficiency is another prerequisite．In order to acquire vocabulary incidentally through extensive reading, a student has to know at least 98％of the lexical items in a reading passage，which implies knowing about 5000 word families or about 8000 lexical items．However，it is sobering to note that most of the students in China have a vocabulary of much less than 1 800 words，which is the requirement by high schools，and those from the remote areas even fewer. Therefore，if we really want to acquire the practical ability to use English as a means to exchange information，we must begin to apply new ways of learning .2．Definition of VocabularyA word(also called a base word or a word family)is defined as including the base form and its inflections and derivatives．Although this definition of a word is convenient and commonly used in vocabulary research，it should be remembered that vocabulary learning is more than the study of individual words. It has been observed that a significant amount of the English language is made up of lexical phrases，which range from phrasal verbs to longer institutionalized expressions.Basically，vocabulary knowledge consists of receptive vocabulary knowledge and productive vocabulary knowledge，so learning a word involves receptive learning and productive learning．Receptive learning refers to being able to recognize a word when it is met．Productive learning involves what is needed for receptive teaming plus the ability to speakor write vocabulary at the appropriate time．It may therefore be most useful to see vocabulary knowledge as a scale running from recognition of a word at one end to automatic production at the other.3. Vocabulary Acquisition through Reading3.1 Vocabulary Acquisition through ReadingReading aids vocabulary acquisition，and vocabulary is predictive of comprehension．It has been consistently demonstrated that reading comprehension is strongly related to vocabulary knowledge，more strongly than to other components of reading．It is argued that we can acquire many words through reading extensively. Through reading，we can not only get the meaning of the target words，but also the knowledge about different aspects of the vocabulary, such as the register, grammatical patterns and collocation and etc．On the other hand，the word variable is more highly predictive of comprehension than of sentence variables，and improvement in reading comprehension can be attributed to an increase in vocabulary knowledge. Conversely, all insufficient vocabulary in English may hinder successful comprehension and the most significant handicap for L2 readers is not lack of reading strategies but failure to achieve automated recognition of L2 word form.3.2. The Role of Context in L2 Vocabulary Acquisitionsome researches that demonstrate the need of context in L2 vocabulary acquisition．For example，long-term results from meaning-focused L2 instructional programs such as immersion indicate that aspects of new vocabulary knowledge can be gained through pedagogy that emphasizes the global comprehension of meaning．What’s more，Hulstijn(1 992)points out that L2 word acquisition does need context．However, the process by which incidental vocabulary learning takes place is quite slow and the amount of vocabulary-acquired through this kind of learning is relatively small. Despite the simple reason that native speakers of English use contexts which they must have fully understood to infer, but making such connections is probably much harder for readers in a foreign language for whom many words in the context may be unknown or only partially known．Therefore，second-language learners will be less effective than native speakers at using context，at least until they achieve a fairlyhigh level of L2 proficiency，that is，successful and efficient incidental vocabulary learning just through reading alone cannot be expected．And an explicit instruction plays a relatively greater role in the vocabulary growth of second—language learners，compared with context．4. Incidental Learning and Intentional Learning4.1 Definition of Incidental Learning and Intentional LearningIn both LI and L2 lexical teaching and learning，there are two types of vocabulary learning：incidental learning and intentional learning．Incidental is defined as the type of learning that is a byproduct of doing or learning something else，whereas intentional learning is defined as being designed，or intended by teacher or students．4.2 Advantages of Incidental LearningIncidental learning，which occurs without the specific intent to focus on vocabulary, has been shown to be an effective way of learning word meanings from context．In the psychological literature，numerous experiments have been conducted to verify the fact that incidental learning plays an important role in L2 vocabulary acquisition of students．For example，Nagy, Herman，and Anderson(1985)，using 1000 word passages and 57 eighth—grade students，have shown that incidental learning accounts for a large proportion of vocabulary growth by school age children. And Anderson(1995)also conclude from their own experimental study that children indeed do learn large numbers of words by means of incidental learning from written context．4．3Limitations of Incidental LearningAcquiring vocabulary incidentally through reading in context fosters an elaborate processing of the word and therefore facilitates its retention in memory. However, guessing from context is a complex and often difficult strategy to carry out successfully．As many linguists put it，acquiring the vocabulary incidentally through reading has the following limitations．Firstly, students have less vocabulary than sufficient for successful incidental learning．Nation and Coady argue that successful guessing in context occurs when about 95％of the lexical items in a text are already known．They point out that this implies knowing about3000 word families or about 5000 lexical items．Presumably the reader would then be an independent learner capable of learning words through context in the same manner as L1 learners．But it is sobering to note that although grasping 1 800 words was the requirement by high schools，when they graduate from the senior middle school，most of the students in China have a vocabulary of much less than this number when they enroll in university．In practice，most students complain that the passages they are asked to read have so high a coverage of unfamiliar words that they have to stop from time to time to look them up in the dictionary，and thus gradually lose the interest of reading.If one does not know enough of the words on a page or have comprehension of what is being read，he cannot easily learn the vocabulary from context.Secondly, inferring word meaning is an error-prone process．Even if one knows 98％of the words in a text，that “unless the context is very constrained，which is a relatively rare occurrence，or unless there is a relationship with a known word identifiable on the basis of form and supposed by context，there is little chance of guessing the correct meaning”(Kelly，1990)．What’s more，recent studies have verified the fact that repeated exposure positively influenced vocabulary learning，but that the relationship was considerably complicated by other factors，such as meaningfulness of the context, similarity of other words in the L1 and incompatibility between the reader’ schem a and the text content．The presence of clues was mainly responsible for students’failure to guess the word meaning successfully from the context．Thirdly, inferring word meaning is likely to be a very slow process。