This study is an attempt to examine the use of linguistic resources by primary science students so as to understand the conceptual and language demands encountered by them when constructing written explanations. The students' written explanations and the instructional language (whole-class discussion and textbook) employed over the topic, the life cycle of plants, in four grade 4 classrooms (age 10) taught by three teachers constitute the data for this study. Students' written explanations were subjected to a combination of content and linguistic analysis. The linguistic analysis was conducted using selected analytical tools from the systemic functional linguistics framework. A diversity of linguistic resources and meanings were identified from the students' explanations, which reveal the extent to which the students were able to employ linguistic resources to construct written scientific explanations and the challenges involved. Both content and linguistic analyses also illuminate patterns of language use that are significant for realising scientific meanings. Finally, a comparison is made in the use of linguistic resources between the students' explanations and the instructional language to highlight possible links. This comparison reveals that the teachers' expectations of the students' written explanations were seldom reflected in their oral questioning or made explicit during the instruction. The findings of this study suggest that a focus on conceptual development is not sufficient in itself to foster students' ability to construct explanations. Pedagogical implications involving the support needed by primary students to construct scientific explanations are discussed.