FORMATIVE REACTIONS 71 



The activity of growth corresponding to a new temperature is 

 rapidly assumed, whereas a change of illumination often acts extremely 

 slowly, the after-effect of the previous conditions persisting for a long 

 time. Sudden changes of temperature, or of concentration, also produce 

 a certain disturbance which may take the form either of a transitory 

 retardation or acceleration of growth, or the one reaction may follow 

 the other. 



In addition to the essential external agencies which regulate growth 

 and render it possible, others may exercise a special stimulating action 

 upon it. This occurs during the reaction to wounding, and when the 

 perennation period is shortened by treatment with chloroform. Chemical 

 stimuli are required to induce the germination of certain seeds and spores, 

 and indeed it is hardly surprising that nutrient materials should act as a 

 stimulus to germination upon the spores of many mould fungi. In such 

 cases, by satisfying special requirements, influences retarding growth are 

 removed. On the other hand, a direct retardation is produced upon the 

 growth of nitrifying bacteria by small traces of sugar, which for most 

 plants is an excellent food-material. 



SECTION 21. Formative Reactions. 



The external conditions influence not only the rapidity of growth 

 but also its character, and frequently in such a manner as to produce 

 pronounced change of shape. Thus the land and water forms of 

 different plants, as well as the stages in the life-cycle of a metoxenous 

 fungus, may differ so greatly as to appear like distinct species to the 

 unscientific observer. Similarly, special external conditions may cause 

 the non-production of certain organs, or the omission of certain stages in 

 the life-cycles of a few plants. Not only may generally distributed 

 influences produce special growths, but also localized ones, as is illustrated 

 by the formation of galls, and by the formation of callus-tissue over 

 wounds. Similarly, the unilateral action of light, gravity, moisture, or 

 oxygen may induce asymmetric growth, and thus lead to dorsiventrality, 

 or to the production of curvatures which are of great importance in the 

 correct orientation of the plant and its organs. 



At the same time the internal structure may also be affected, and 

 indeed internal formative changes may occur subsequently to the attainment 

 of adult shape 1 , as, for example, when an internal injury is healed in an 

 adult organ, or when an increased stress causes a thickening of the cell- walls 

 in the mechanical tissues of a stem, or when an increase of transpiration 



1 Numerous instances are given in Goebel's Organography, Clar. Press, 1900. Cf. also 

 Schumann, Bot. Centralbl., 1891, Bd. XLV, p. 357. 



