INFLUENCE OF GROWTH SUBSTANCES ON GROWTH AND CELL DIVISION IN GREEN ALGAE

THE FIRST report of the effect of growth substances on algae was made by Yin (1937). In his tests with Chlorella vulgaris he found no measurable increase in the rate of multiplication, but he did find slight increases in size of cells in cultures treated with heteroauxin. At about the same time a report of the effect of heteroauxin on Chlorella miniata, C. pyrenoides (pyrenoidosa ?), Cystococcus cohaerans, Oocystis naegeltii, and Scenedesmus flavescens was published by Leonian and Lilly (1937). They concluded that heteroauxin is a growth-inhibiting rather than a growth-promoting substance so far as concerns the organisms they studied. Evidence was offered in a preliminary report (Brannon, 1937) of the stimulatory effect of growth substances on Chlorella vulgaris, C. pyrenoidosa, and Oocystis sp. Pratt (1938) offered additional evidence to show that chemical growth substances, including heteroauxin, cause an increase in the rate of multiplication in Chlorella vulgaris. He, however, was unable to confirm Yin's statement that heteroauxin causes an increase in size of cells. Growth substance studies with vascular plants have probably been emphasized in the past because in these plants correlations, formation of organs, tropisms, and modifications of growth may be studied to better advantage than in algae. In dealing with vascular plants the concentration of growth substance must vary as the solution diffuses through tissues intervening between the point of application and that at which the effects of the applied growth substance are to be studied. This view is supported by Hamner and Kraus (1937) in connection with their histological study of bean plants treated with applications of indole-acetic acid in lanolin. If, instead of using lanolin, the growth substances are injected into the underlying tissues or applied superficially after removing the overlying tissues, the factor of traumatism is introduced. On the other hand, unicellular algae may be immersed in solutions containing known concentrations of growth substance, and the results of such treatment should indicate the effects of the concentration in question. In both cases, however, the concentration of growth 1 Received for publication October 21, 1938. The work reported in this paper was conducted for three years in the botanical laboratories of the University of Wisconsin. Grateful acknowledgment is made of the many courtesies received from Dr. E. M. Gilbert, chairman of the Department of Botany. Dr. A. J. Riker, Dr. Winston Manning, Dr. Charles E. Allen, and Dr. John T. Curtis also gave many helpful suggestions and criticisms. Ne are indebted to Dr. P. W. Zimmerman of the Boyce Thompson Institute for Plant Research for valuable advice and for the initial growth substances used. The Wisconsin Alumni Research Foundation generously supported this work with grants for two years. substances which enters into the cells is still unknown. Bonn r (1934) has pointed out that the magnitude of growth substance effects is directly related to the hydrogen-ion concentration of the medium in which the growth substance is applied. He concluded from his study with the Avena coleoptile that the growth substance is converted from the form of an inactive salt to an active nondissociated form in the presence of acid buffers. Marmer (1937) also found that the effect of growth substances on the growth of wheat seedlings is related to the pH of the applied solution. She reported that indoleacetic acid is 15,000 times as effective at pH 4.6 as at pH 7.5, indole-butvric acid is 10 times as effective, and indole-propionic acid 20 times as effective. Albaum, Kaiser, and Nestler (1937) found that the penetration of indole-acetic acid into cells of Nitella takes place more rapidly at pH 3.65 than at pH 7.94. On the basis of their results they concluded that the hydrogen-ion concentration affects the degree of dissociation of molecules of growth substance and that heteroauxin enters cells of Nitella in molecular form. Since in algal cultures it is possible to adjust the pH to a definite value, the effects of known concentrations of growth substance may be studied and compared to greater advantage than in the case of vascular plants where the hydrogen-ion relation may become a variable factor. The present study was undertaken to determine the effect of several growth substances on cell division and cell size in three species of unicellular green algae. MATERIALS AND METHODS.-Three types of algae which have different growth rates under identical conditions were selected for this study. Chlorella vulgaris Beyerinck was selected as representative of the fast-growing, Coccomyxa simplex ( ?) of the intermediate, and Mesotaenium caldariorum (Lagerh.) Hansg. of the slow-growing forms. The first named species was selected in accordance with Du Buy's (1936) statement that "it has been proved previously that growth regulators of the auxin type have an effect only on cells which are in the embryonic stage." Went and Thimann (1937) also concluded that "the cells on which the auxins act are of the least differentiated and simplest type." The unicellular algae which possess a stage corresponding to embryonic cells of higher plants are those which, as does Chlorella, reproduce by means of autospores. These spores have exceedingly thin walls and are highly responsive to physical and chemical changes in the medium in which they are immersed. These algae were identified in the Pflan-