. Contributions from the Laboratory of Plant Genetics. July, 1924] BRINK — PHYSIOLOGY OF POLLEN 421 was not secured on each medium. In a considerable number of cases, however, good growth on both blocks of the pair was secured. On pairs of blocks containing the respective concentrations of sugar, 0-3, 3-10, 0-10, 0-20, 3-20, and 10-20, the pollen tubes of Cucumis, Primula, Hippeastrum, and Nicotiana grew entirely at random. Differences in concentration of sugar in the adjacent areas occasioned no orientation of the tubes. Simi- larly, on pairs in which the differential was sterile yeast, the t
. Contributions from the Laboratory of Plant Genetics. July, 1924] BRINK — PHYSIOLOGY OF POLLEN 421 was not secured on each medium. In a considerable number of cases, however, good growth on both blocks of the pair was secured. On pairs of blocks containing the respective concentrations of sugar, 0-3, 3-10, 0-10, 0-20, 3-20, and 10-20, the pollen tubes of Cucumis, Primula, Hippeastrum, and Nicotiana grew entirely at random. Differences in concentration of sugar in the adjacent areas occasioned no orientation of the tubes. Simi- larly, on pairs in which the differential was sterile yeast, the tubes appeared to grow entirely at random. Using Pfeffer's method of fine capillary tubes rilled with the solution being tested and closed at one end with paraffin to retard diffusion, marked chemo- tropism of Narcissus pollen tubes for sodium malate has been found. Further experiments are in progress with salts of malic Text Fig. 3. Pollen tubes of paper-white Narcissus, showing chemotropism for an ovule on an artificial medium. It is interesting to note, in connection with text figures I, 2, and 3, that the pollen tubes of Narcissus are attracted not only by the ovules but also by the placenta and by pieces of the epidermis from the internal wall of the carpel. A quantitative method of determining the relative chemotropic values of these different tissues has not yet been devised, but it is quite obvious that, unless the ovule exerts a greater attraction to pollen tubes in the ovarian cavity than other parts, chemotropism can play no role in directing the pollen tube to the micropyle. The Effects on Growth of Grouping Grains and the Problem of Self-sterility Early in our work it was observed that tubes arising from groups of grains on artificial media were longer than those produced by single isolated grains. Attention was first directed to the condition in cultures of Nico- tiana, where it regularly appeared that the longest tubes came from large. Please note that th
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