nearby lateral buds. Or a low concentration 

 of an auxin may merely accelerate the elon- 

 gation of the cells of a growing stem, whereas 

 a higher concentration stimulates cell mul- 

 tiplication and leads to the putting forth of 

 roots from the stimulated section of the stem. 

 As yet, no precise information is available in 

 regard to how the auxins produce their ac- 

 tion. A promising hypotheses is, however, that 

 auxins exert their influence through certain 

 metabolic enzymes — which would account for 

 the fact that a very minute quantity of each 

 compound suffices to give a very large effect. 



Other Phytohormones. Gibberellic acid 

 (CjyH^oO,;) and several other closely related 

 compounds constitute another very interest- 

 ing group of plant hormones. These, collec- 

 tively, are designated as the gibberellins. The 

 gibberellins began to be recognized by Japa- 

 nese workers as early as 1926. These workers 

 studied a disease of rice, called the "foolish 

 seedling disease," so named because it causes 

 young rice plants to grow too rapidly and to 

 assume grotesquely tall, abnormal forms. The 

 disease is caused by a fungus (Cribberella 

 fujikuoi) infection. This fungus, in fact, pro- 

 vides a source from which several active 

 compounds have been extracted and iden- 

 tified. 



The gibberellins not only stimulate very 

 rapid growth in young stems, but they also 

 induce the development of flowers in a num- 

 ber of plants, even when normal flower-in- 

 ducing stimuli (p. 266) are withheld. It seems 

 likely, therefore, that further study will show 

 that gibberellins may play an important and 

 interesting role in plant development and 

 physiology. 



Some other substances that appear to be 

 important as plant hormones are: (1) diphen- 

 ylurea and other factors extractible from 

 coconut milk; and (2) kinetin, and other 

 adenine derivatives, which have been ex- 

 tracted from yeasts and other plant materials. 

 The coconut milk factors stimulate cell 

 growth and cell division in many plant 

 embryos. Kinetin, on the other hand, appears 

 to influence bud development. Plant shoots, 



The Responses of Higher Plants - 265 



grown in culture medium, respond by put- 

 ting forth more and more roots as kinetin is 

 replaced by auxin. 



Plant Inhibitors. A relatively recent field 

 in plant physiology was opened up by the 

 discovery of certain very potent inhibiting 

 substances. These provide a system of checks 

 and balances in the operation of the phyto- 

 hormone system. Particularly promising in 

 this regard is a group of unsaturated lactone 

 compounds, for example, coumarin and 

 parascorbic acid. Root growth, seed germina- 

 tion, and floral development have all proved 

 susceptible to inhibition by one or another 

 of these substances. In fact, a fuller under- 

 standing of the mode of action of plant hor- 

 mones probably will involve further knowl- 

 edge about inhibitors. 



Photoperiodism and Other Responses to 

 Light. A currently active field of research 

 deals with the triggering of plant responses 

 by seasonal changes in their exposure alter- 

 nately to daylight and darkness. The flower- 

 ing response has been studied most inten- 

 sively. However, considerable attention has 

 also been given to the germination of seeds 

 and to early embryonic growth. 



The regularity with which various plants 

 produce their flowers on a well-regulated 

 seasonal schedule is familiar to everyone. 

 However, only recently, starting in 1920 

 with the work of Garner and Allard of the 

 U.S. Department of Agriculture, has it been 

 realized that this nicely timed schedule of 

 flowering is governed, in a majority of cases, 

 by seasonal changes in the length of the 

 nights and days. 



This work has made it possible to classify 

 flowering plants in three categories: (1) short- 

 day species, such as asters, chrysanthemums, 

 and soybeans, which normally bloom at a 

 specific time in the late summer or fall, when 

 the nights are growing longer; (2) long-day 

 species, such as clover, beets, and delphinium, 

 which bloom in spring or early summer, 

 when the period of darkness is growing 

 shorter; and (3) indeterminate species, such 

 as carnations, dandelions, and tomatoes, 



