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Internode growth of vegetative Xanthium plants was studied at various levels on the stem with application of the plastochron index. The bottom internodes were of small size and they displayed small rates of growth. Both, the size of the internodes and their growth rates increased proceeding in the acropetal direction. Rates of cortical cell elongation followed a bell shaped curve, starting with a rate of 7 µm per day, reaching a peak of 15 µm per day and stopping after leaf plastochron index 14. Young internodes, smaller than 25 mm, grew at a constant relative elemental rate of 0.2 day-1 throughout. Internodes larger than 30 mm displayed an acropetal pattern of elongation.

Effects of the amino acid analog D,L-p-fluorophenylalanine on auxin oxidizing activity from Avena saliva cv. Clintford have been examined. Incubation of etiolated Avena coleoptile apices with D,L-p-fluorophenylalanine, previously reported to promote elongation and depress phenolic metabolism, lowers auxin oxidizing activity. Temperature and pH profiles demonstrated that depression of oxidizing activity by D,L-p-fluorophenylalanine occurs over a relatively wide range. Incubation of coleoptiles in media containing L-phenylalanine alone or L-phenylalanine combined with D,L-p-fluorophenylalanine result in oxidizing activities which are, respectively, slightly higher than or restored to control levels. Thus, dark incubation of coleoptiles with D,L-p-fluorophenylalanine results in auxin oxidizing activity which differs from that of control and other treatment groups. Oxidizing activity from D,L-p-fluorophenylalanine pretreated coleoptiles has a Michaelis constant (70 µM) and a maximum velocity (0.200) which are consistently lower than the control constants. These findings provide further clarification of the effects of D,L-p-fluorophenylalanine, on coleoptile elongation and auxin oxidizing activity in etiolated Avena coleoptiles.

When Xanthium lamina width is plotted versus leaf length during the entire period of growth, a straight line is obtained representing an allometric relationship with a regression correlation coefficient of 0.9973 a lamina width to length ratio of 0.502 ± 0.01 and chordate morphology. On the other hand, gibberellic acid treated plants yield a correlation coefficient of 0.9871 a lamina width to length ratio of 0.372 ± 0.0074 and lanceolate leaves. The fraction of leaf width to leaf length is a measure of lamina width reduction. Gibberellic acid alters the mechanism which controls the balance between the leaf length and the leaf width.

Serial sections of mature stems of ten species of herbaceous dicotyledonous plants were examined by light microscopy and the number of chloroplasts per cell was estimated in epidermis, collenchyma and cortex. Chloroplast identification was made by both light and transmission electron microscopy. Chloroplasts were present in epidermis, collenchyma and cortex tissues of all stems examined. The smallest number of chloroplasts was observed in the epidermis. Collenchyma cells had the largest number of plastids in four of the genera and cortex cells had the largest number in the remaining six genera. The stem epidermis of all genera contained stomates as demonstrated by scanning electron microscopy and aceto-orcein stained epidermal peels.

Background and Aims: Leaf morphology, anatomy, degree of lignification, and tensile strength were studied during vegetative phase change in an inbred line of Zea mays (OH43 x W23) to determine factors that influence mechanical properties during development. Methods: Tensometer, light microscopy, histochemistry. Key results: Mature leaf length increased linearly with plant development, peaked at leaves 7 and 8 (corresponding to the onset of the adult phase) and then declined. Leaf width was stable for leaves 1 through 3, increased to leaf 7, remained stable to leaf 10, and then declined through leaf 13. Lamina thickness was highest for leaf 1 and decreased throughout development. Leaf failure load to width ratio and failure load to thickness ratio increased with development suggesting that changes in leaf morphology during development do not entirely account for increases in failure load. Histochemical analyses revealed that leaf tensile strength correlates with percent lignification and the onset of anatomical adult features at various developmental stages. Conclusions: These data demonstrate that in Zea mays lignification of the midrib parenchyma and epidermis may be directly correlated with increased tensile strength associated with phase change from juvenility to adulthood. Failure load and resultant tensile strength values are primarily determined by the percent tissue lignification and the appearance of leaf architectural characters that are associated with the transition from the juvenile to the adult phase. Increased mechanical stability that occurs during the phase transition from juvenility to adulthood may signify a fundamental change in strategy for an individual plant from rapid growth (survival) to reproduction.

Division and growth of chloroplasts was studied during leaf development of Xanthium pensylvanicum at various stages of development represented by the leaf plastochron index.Between leaf plastochron indices -1.00 and 2.56 chloroplast division was observed with little enlargement. Between 2.50 and 5.00 chloroplasts enlarged in diameter with an average rate of 0.21 µm per day. At leaf plastochron index 5.00 chloroplasts attained their mature size of 6.12 µm. No chloroplast division was found after leaf plastochron index 2.50. A change in shape of plastids from spherical proplastids to discoidal accompanied their growth during stages 2.50 and 5.00.

Vegetative Xanthium plants grown under noninductive conditions were marked with India ink along the stem and photographed at three consecutive time intervals. Relative elemental rates (d(dX/dt)/dX) of stem elongation were estimated from displacement of marks during stem elongation. Young internodes elongated with constant relative elemental rates of 0.2 day to the -1. Older internodes displayed an acropetal pattern of elongation in which the basal segments of an internode stopped elongating first and the apical portion last. Nodal regions elongated with very small relative elemental rates of 0.05 day to the -1. Rates decreased as the age of the internodes and nodes increased and stopped shortly after Leaf Plastochron Index 9.