What do neighboring cells ‘say’ to one another? The answer is, of course, that we do not know. It is often accepted as self evident that local relations between cells must have a major role in organized development. It is assumed that signals passing between cells must be varied and specific, being responsible for the diversity of cell types. These assumptions require proof. The problem of obtaining evidence is that whatever the events, they occur over minute distances and are not readily amenable to experimental manipulations. Of course, the correlated differentiation of neighboring cells is itself evidence for interactions, but these correlations can often be understood without assumptions of local, specific interactions (Chapters 6–8, 10).

Additional evidence can be gleaned from various approaches. This chapter more than any other must therefore have a review aspect and deal with different topics, related to one another primarily by their implications for the general subject of cellular interactions. Thus, the microscopic examination of plant tissues reveals various structures that span two or more neighboring cells. Such continuity, the correspondence of special locations in different cells, could not be due to chance. The occurrence and development of the most common of these structures will therefore be considered. Another, related topic will be the developmental changes which occur when tissues are experimentally brought into new varied contacts, when originally separate cells are grafted together.