Growth of the Lilium longiflorum pollen tube in vitro is restricted to a zone extending back 3–5 μ from the tip. Electron micrographs of cross and longitudinal thin sections of L. longiflorum and L. regale pollen tubes reveal that the cytoplasm of the nongrowing region of the tube contains an abundance of mitochondria, amyloplasts, Golgi bodies, endoplasmic reticulum, lipid bodies, and vesicles. In contrast, the growing tip is characterized by an abundance of vesicles and an absence of other cytoplasmic elements. The vesicles appear to be of 2 types. One is spherical, about 0.1 μ in diameter, stains strongly with phosphotungstic acid, apparently arises from the Golgi apparatus and appears to contribute to tube wall and plasmalemma formation. The other type is irregular in shape, 0.01‐0.05 μ in diameter, stains strongly with lead hydroxide, and is of unknown origin and function. Cytochemical analysis indicates that the tips of L. longiflorum pollen tubes are singularly rich in ribonucleic acid, protein, and carbohydrate. These findings are discussed in relation to tube growth.

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Bleaching (interference with chlorophyll accumulation) by streptomycin (SM) is more effective when dark‐grown Euglena gracilis cells are exposed to SM in the dark for several days than when exposure of dark‐ or light‐grown cells is exclusively in light. Cells are also bleached when exposed to SM while lacking both chlorophyll and chloroplasts (i.e., exposed to SM in the dark and subcultured to SM‐free medium before transfer to light). Bleaching therefore involves inhibition of the formation of a plastid or pigment precursor, rather than breakdown of chlorophyll or interference with replication of mature chloroplasts. Bleaching by SM B prevented by addition to the medium of any of several fin cations, including Mg. Therefore it is proposed that bleaching results from chelation of Mg by SM. thus blocking a Mg‐requiring step in chlorophyll synthesis. Disruption by SM of chloroplast structure and/or development is considered a secondary consequence of the bleaching action, resulting from the essentiality of intact chlorophyll molecules for normal chloroplast structure.

We found that the auxin-induced growth is mediated through the activation of the dictyosomes (collectively, the Golgi apparatus). Incubation of oat (Avena sativa) coleoptile segments in indoleacetic acid-sucrose-phosphate buffer changes significandy the number of dictyosomes in the expanding cells. A further indication of auxin enhancement of dictyosome activity is a decrease in dictyosomal cisternae (flattened membranous sacs) number. This decrease occurred after 6 minutes of incubation in auxin, and then was followed by a reduction in the organelle number per se. These times are in keeping with the rapid action of auxin-induced cell elongaton, and the latent period of geotropism. In the apical cells, the effect of indoleacetic acid is more subtle and complex. The 3To whom reprint requests should be addressed.Individual stacks of cistemae (flattened sacs containing fluid). The association of dictyosomes within a cell forms the Golgi apparatus (14). regulated by the hormone auxin. In this study we ask whether auxin, IAA, changes the activity of the dictyosomes. And if so, are the kinetics of the changes in keeping with those of the tropistic and growth responses. We examined the quantity and activity of this organelle in the extreme apex as well as in the elongating cells of oat coleoptiles after different times of incubation in IAA solutions. MATERIALS AND METHODSPlanting and Incubation. Oat seeds (Avena sativa, Victory I) were soaked in warm tap water (initially 40 C) for 2 hr, drained, and kept in the dark for 20 hr at 4 C. The soaked seeds were then aligned, embryo up, on lucite bars covered with moist filter paper. The seeds were exposed to red light (G.E. Ruby Red tungsten filament lamp, 10 w) for 24 hr, and grown for an additional 48 hr in the dark. At the end of this period, the coleoptiles reached a length of 2 to 3 cm. The apical 1 mm, and the subapical 3 mm (4th through 6th mm) of the coleoptile were sectioned with spaced razor blades. The segments were placed in Petri dishes containing buffer, 0.1 M KH2PO4 and 2% sucrose (pH 4.6). The dishes were set on a slow reciprocating horizontal shaker for a minimum of 2 hr, in order to deplete the tissues of their endogenous auxin. The segments were then transferred to small culture dishes containing the same buffer solution with or without 1 ,M IAA (sodium salt). The dishes were then placed on the shaker, and the segments were harvested at 6-min intervals for a period of 60 min for examinations by EMS and for the determination of cell lengths. All manipulations were carried out in a dim green light (21) at 25 C and 70% relative humidity.Electron and Light Microscopy. At 6-min intervals, the segments (n = 6) were transferred to a cold fixative (approximately 4 C) containing 4% glutaraldehyde, 1 % acrolein buffered with 0.1 M Na-cacodylate to pH 7.4, and 0.1% CaCI2. Fixation was carried out in a refrigerator (4-6 C) overnight. The aldehyde-fixed segments were then washed with the same buffer in which they were fixed, and postfixed for 3 hr in cold 1 %...

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