It is widely known that, for a better teaching of science across al education levels, inquiry-based approaches produce better results in content knowledge achieved by students. As a first step, these methodologies require that teachers can identify the previous ideas that students present on a topic before its formal teaching. In this sense, when children arrive at school, they present "previous knowledge" that comes from the sensorial perceptions of the facts of life or through the processes of socialization. Children establish their own explanations of reality, but these do not always present a scientific basis. These are conceptual schemes that are alternative to scientific knowledge, deeply rooted on children cognitive structure and with a very important level of internal coherence. As a consequence, these errors are often difficult to modify and these may prevail in adult ages. So, it is important to know what students have in mind before beginning the teaching/learning process because it may provide significant data when selecting the teaching objectives. This research approach about alternative conceptions is based on the constructivist paradigm of learning, that is to say, previous knowledge and conceptions of students interfere and affect their learning in new contexts. For this reason, the objective of this work is to know the alternative conceptions that a group of 52 students of third-grade of Primary Education in Spain (8-9 years-old) have on reproduction of plants. This research was carried out using two field instruments: a questionnaire with eight questions and interviews. Questionnaires were delivered to all the students, whereas the interviews were carried out on a total of seventeen questionnaires that required to expand and to justify the answers.

Results of this study show the following common ideas of children about plant reproduction:
1. Children have a vision of reproduction in plants influenced by how reproduction in humans and animals is (they think about pregnancy and coitus);
2. Plants do not breed but they are born by seeds, which are small miniatures of plants;
3. Birth and life of plants is directly related to the existence of human beings and so, people are always who cultivate and take care of plants;
4. The function of flowers in plants is ornamental and to feed other animals, such as birds and insects.

These ideas hold by early Primary Education students should be taken into account when developing inquiry-based sequences of activities, which should provide students the necessary opportunities to overcome these non-scientific ideas and to generate scientific ideas according to the scientific practices.