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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.

Compatible and incompatible pollen tubes growing on detached Lilium longiflorum pistils which had been prelabeled with myoinositol-U-14C take up a portion of the label and utilize it for biosynthesis of tube wall substance. The label is transferred from pistil to pollen tubes apparently via the secretion products (exudate) of the pistil.The exudate thus appears to have a major nutritional role in pollen tube growth in vivo.Pollen tabes produced in vitro seldom achieve the length needed for fertilization in vivo. One may assume that pollen tubes growing in vivo obtain additional substances necessary for their development from the pistil. A study of the nutritional role of the pistil in pollen tube development is described in this paper. We were interested especially in the utilization of pistil material for tube wall synthesis. Myoinositol, a precursor of uronosyl and pentosyl units of cell wall polysaccharides of higher plants, is utilized by detached Lilium longiflorum pistils for cell wall polysaccharide formation and for biosynthesis of exudate, the secretion product found on the stigma and in the style canal of pollinated pistils (2). As pollen tubes grow through the style canal, they are surrounded by this exudate, which may provide the tubes with carbohydrate material for wall synthesis. To explore this possibility, we have pollinated pistils that were previously labeled with myoinositol-U-'4C and examined the growing pollen tubes for uptake and incorporation of label into tube wall substance.

The fine structure of the cells which line the canal of Lilium longiflorum pistils confirms the secretory function which has been ascribed to them. The cells differ in structure from the secretory cells which cover the stigma surface and are therefore referred to as canal cells rather than stigmatoid cells. Their most striking feature is an elaborate wall, 8–14 μ in maximum depth, on the side facing the canal, which with associated structures, we term the secretion zone. Pollination, which triggers chemotropic activity in the style and secretory activity in the canal cells, is not correlated with marked changes in the fine structure of the canal cells. The canal cells appear to fit well into that category which Gunning and Pate have termed “transfer cells.”

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