Histological Reactions of Bean Plants to Growth Promoting Substances

Abstract

1. Apical tumors developed following the application of 3 per cent indoleacetic acid lanolin mixture to cut surfaces of decapitated bean plants may continue development for periods longer than six months and attain diameters of 2 cm. or more. Such tumors are developed largely from proliferations of the cells composing the pith, some of which differentiate into vascular elements variously arranged and anastomosed; others remain more or less actively meristematic. 2. Large, irregular vascular tumors are developed when similar mixture is applied as a narrow band encircling an uninjured young internode of the stem. Response is about equal above and below the band under average greenhouse conditions. 3. The stems to which such bands have been applied when grown under average greenhouse conditions are not retarded in their apical extension but grow and fruit as well as untreated stems under comparable environmental conditions. The pith of stems treated in this manner shows virtually no meristematic response, but develops as does that of stems which have been untreated. The roots developed are related to the rays in the same manner as those developed in apical tumors of decapitated plants. 4. There are more or less definite rhythmic periods of rapid development and comparative quiescence in the development of lateral tumors through an extended period. The irregular lateral outgrowths or tubercles of lateral tumors are developed mainly from derivatives of the phloem and to some extent from the cortex. 5. Apical tumors resulting from the application of 1.5 per cent indolebutyric acid in lanolin to decapitated stems were similar to those resulting from applications of 3 or 1.5 per cent indoleacetic acid mixture. They differ mainly in having somewhat fewer, thicker roots in the upper zone, more roots produced 1 cm. or so below the treated surface, and several tiers of roots developed between these two regions. There is slightly greater delay in the formation of the apical tumor above the treated surface, but eventually it develops to as large size, although its surface is generally smoother and less tuberculated than that developed following treatment with indoleacetic mixture. There is marked swelling of the stem and differentiation of vascular strands for several centimeters below the treated surface. 6. Apical tumors resulting from the application of 0.5 per cent naphthaleneacetic acid in lanolin differed from those following the application of the other two substances in having greater, more uniform diameter for a distance of about 1 cm. below the cut surface, where a circle of large root primordia is formed, each primordium over one of the main vascular bundles. Below these primordia the tumor tapers sharply to a much smaller diameter, giving it a distinct shouldered appearance. Below it prominent ridges extend for several centimeters down the stem. There is much less proliferation of the pith; only a slight tumor is developed above the surface of application. 7. Apical tumors resulting from the application of 1.5 per cent mixtures of each of the substances to individual stems showed greater differences than tumors produced following the application of weaker or stronger concentrations of the same substance. 8. Applications of 3 per cent indoleacetic mixture to the cut surfaces of partially mature pods which had their tips removed resulted in the production of very large vascular apical tumors and roots. The tissues composing such tumors were derived mainly from the proliferated exocarp and mesocarp of the pod. The endocarp which in untreated pods is principally derived from proliferated epidermal cells becomes somewhat more active in treated pods, but forms little or no part of the apical tumor; nor have vascular elements differentiated from this proliferated tissue been observed. 9. The large apical tumors are built up from derivatives of cells which originally were near to or but a relatively few cell layers away from the treated surface, a far shorter distance than is the case in stems. 10. In at least half the apically treated pods the seeds abort. If the seeds continue to develop, the apical tumor produced is generally smaller than if they abort. 11. When the epidermis at the adaxial suture of partially developed pods is slightly abraded and 3 per cent indoleacetic lanolin mixture applied, large vascular tumors are developed from the proliferated exocarp and to a far lesser degree from the mesocarp. In such pods most of the seeds abort. 12. The quantitative response made by stems or pods when treated with any one of the three substances used in mixture with lanolin was influenced more pronouncedly by small variations in the environment or the state of vigor or development of the plant than by comparatively great variations in the concentration of the particular mixture used or in the amount of the mixture applied. There was less difference in this respect in the qualitative than in the quantitative response. 13. Caution is required, therefore, in any endeavor to make a quantitative estimate of the amount of any of these substances which may be present in the living tissues of the plant if such estimate is based on a quantitative response of tissues in close proximity to the place of application or distant from it.