THE PREDOMINANT ROLE OF THE PITH IN THE GROWTH AND DEVELOPMENT OF INTERNODES IN LIQUIDAMBAR STYRACIFLUA (HAMAMELIDACEAE) .2. PATTERN OF TISSUE STRESS AND RESPONSE OF DIFFERENT TISSUES TO SPECIFIC SURGICAL-PROCEDURES

Isolated pith segments when freed of the restraint of contiguous vascular tissue during different stages of internode development expand rapidly, whereas, peripheral tissues when separated from each other immediately shrink, confirming the existence of physical stresses first described by 19th century botanists. Evidence is presented that the rapid production and differentiation of vascular tissue in elongating internodes of Liquidambar contribute significantly to the development of compressive forces in the pith by restraining the growth of pith cells. Heretofore, the role of tensile stresses affecting the growth of internodes in herbaceous plants has been ascribed only to the less extensible outer peripheral tissues, especially the epidermis. Different surgical procedures applied to elongating internodes, including disruption of the central core of pith cells without disturbing the vascular and other peripheral tissues, or conversely, injuring or removing portions of the outer tissues leaving the pith intact, confirm the histological observations that the developing pith is the driving force for internodal elongation.