THE question of why some plants exhibit copious exudation and others show little or none has remained unanswered. Very few comparative investigations of variations in root pressure among species, however, have been carried out. Reports of root pressure exudation have been based on independent observations of a wide range of materials, from cut stumps of large trees1,2 to small root segments raised in sterile culture3. Assumptions about the variable root pressure exudation among species have been based on data obtained using such a wide range of plant materials and techniques. An example is the long held assumption that conifers rarely, if ever, develop root pressure. Yet, when excised apical segments of actively growing conifers roots were observed, exudation was always present4. Similarly, it was observed that fully suberized roots would exhibit significant amounts of exudation when individual roots were studied under controlled conditions4,5. In fact, suberized roots showed higher rates of exudation than non-suberized roots from the same plant. For a study of comparisons of root pressure exudation among species, it seems, therefore, that similar techniques and plant tissues must be used.