234 BOTANY 



the organ, generally at its base (leaves and flower-stalks of many Monocotyledons), 

 grand periods of growth are also apparent. A shoot of Phaseolus multiflorus 

 which was divided, from the tip downwards, into transverse zones 3 '5 mm. broad, 

 showed in forty hours, according to Sachs, 



in zones: I., II., III., IV., V., VI., VII., VIII., IX., X., XL, XII. 

 an increase of 20, 25, 45, 65, 55, 30, 18, io, io, 5, 5, 5 

 tenth-millimetres. 



This periodicity in the growth in length occurs even when the external influences 

 affecting growth remain unchanged, and is determined by internal causes alone. 



External Influences upon Growth. — External factors often take 

 an active part in the process of elongation, either as retarding or 

 accelerating influences. As growth is itself a vital action, it is affected 

 by any stimulus acting upon the protoplasm ; on the other hand, as it 

 is also a physical function, it is modified by purely physical influences. 

 Growth is particularly dependent upon temperature, light, moisture, the 

 supply of oxygen, and the existence of internal pressure and tension. 



The influence OF TEMPERATURE is manifested by the complete 

 cessation of growth at a temperature less than 0° or higher than 

 40°-50°. Between the minimum and maximum temperatures, at which 

 growth ceases, there lies an OPTIMUM temperature (p. 163), at which 

 the rate of growth is greatest. This optimum temperature usually lies 

 between 22° and 37° C. The three cardinal points of tempera- 

 ture here given include a wide range, as they vary for different 

 species and even for individual plants of the same species. In tropical 

 plants the minimum temperature may be as high as + 10°C, while 

 those of higher latitudes, where the first plants of spring often grow 

 . through a covering of snow, as well as those of the higher Alps and 

 polar regions, grow vigorously at a temperature but little above zero. 

 In like manner, the optimum and maximum temperatures show great 

 variation in different species of plants. The optimum does not usually 

 lie in the middle between the minimum and maximum, but is nearer 

 the maximum. 



The INFLUENCE OF LIGHT makes itself felt in a different manner from 

 changes of temperature. Light as a general rule retards growth. This 

 is apparent from observations on stems and roots grown in the dark, and 

 is also true in regard to the growth of leaves, if the disturbing effects 

 resulting from long-continued darkness be disregarded. Too great an 

 intensity of light causes a cessation of the growth of an organ, while 

 feeble illumination or darkness increases it. The effect of darkness 

 upon the growth of plants is, however, differently manifested according 

 to its duration, whether it be continuous, or interrupted, as in the 

 changes of night and day. Long- continued darkness produces an 

 abnormal growth, in that the growth of certain organs is unduly 

 favoured, and of others greatly retarded, so that a plant grown 

 altogether in the dark presents an abnormal appearance. The stems 

 of Dicotyledons, in such case, become unusually elongated, also soft and 



