EXPLICATIVE ILLUSTRATIONS TEACH DIRECTLY by explaining and clarifying what might otherwise be difficult or impossible to express in words. Focus first on function, and form will follow - is perhaps what Duchastel and Walker had in mind when they suggested a new "gammar" for illustrations (Duchastel,P.C., & Walker,R. 1979) . This article will focus on the verbs, that is, the functional roles of illustrations that explain in detail. Specifically, the following seven functional roles of explicative illustrations will be described: Descriptive, Expressive, Logico-mathematical, Data-display, Algorithmic, Functional and Constructional.

A drawing or a picture is sometimes the most effective tool for overcoming the abstraction of certain topics in the sciences. For example, how would you describe what a human cell looks like? Small and roundish yes, but then, so is a golf ball. The descriptive picture in Fig. 1a. provides a hollistic impression of a human cell. This image reveals exactly what a human cell looks like. The concept map in Fig 1b. portrays spatially, the various domains of the human cell concept. 

Fig. 1a,.b. Point To The Human Cell To See A Concept Map.

Illustrations are often employed to make an impact on a reader beyond a simple description. A picture of a serious auto accident, for example, could help a student driver understand a painful consequence of drinking and driving. An illustration that evokes an emotional connection between the reader and the subject/topic is performing the expressive role.

Illustrations such as a line graph can be used to display numerical relationships or mathematical concepts. For example,the shape of a line graph reveals whether the variables are related linearly or in some other manner. Ilustrations showing numerical sequence and pattern are performing the logico-mathematical function and make visualization of a trend possible.

Tables, charts, and graphs are commonly found next to prose descriptions in modern science textbooks. Illustrations performing the data-display role facilitate easy access to the data for quick visual comparisons.

If, when, unless, and then, are complex conditionals that can often be simplified through well designed illustrations. A flowchart of cellular differentiation for example, could begin with an undifferentiated stem cell and progress through each stage of cellular development. An illustration is performing the algorithmic role when it displays the entire range of action possibilities.

A large part of science instruction has to do with the organization of systems and the discussion of processes. Although intangibile and often invisible, processess such as cellular division come to life through illustrations that help the learner visualize what could not otherwise be seen. Mitosis, the intangibile process of cellular division is "unfolded" through the illustration provided in Fig. 3. 

Fig. 2. Mitosis: Click on the Cell to.See The Process of Cellular Division.

"The ankle bone's connected to the... shin bone " is a little ditty that expresses the idea behind the constructional role of illustrations. Illustrations can reveal the anatomy of a concept by showing how each component part is connected to form the whole.

The selection of explicative illustrations based upon their functional role has become yet another facet of the educational technologist's area of expertise. The seven functional roles of explicative illustrations presented here are largely distinctive and can serve as a framework upon which instructional effectiveness is improved.

Michael T. Murphy
Graduate Student
San Diego State University