262 BOTANY tart i 



of the stem, remain always appressed to the soil. It determines and 

 regulates the penetration of many tubers and bulbs to a definite depth 

 in the soil. It increases the fixation of the plant in the soil, since 

 greater stalality results from tense than from slack roots. 



External Influences upon Growth. — External factors often take 

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

 accelerating influences. Growth being itself a vital action, it is aftected 

 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 other substances. It is also in- 

 fluenced by pressures and tensions and by injuries. 



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. 178), at 

 which the rate of growth is greatest. This optimum temperature 

 usually lies between 22° and 37° C. The three cardinal points of 

 temperature here given iticlude a wide range, as they vary for 

 different species and even for individual plants of the same species or 

 for different developmental stages of the same individual and for their 

 several vital processes. 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. Many of our spring plants show 

 that the opening of their flowers can take place at a lower temperature 

 than the unfolding of the foliage leaves. 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 maximiun. (On the 

 dependence of the cardinal points on external conditions cf. p. 178.) 



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 dis- 

 turbing 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 in most 

 organs accustomed to light. Tims, in the diminished light of a 

 wood leaves are often larger than in full daylight. 



As a rule, therefore, the growth of plants is greater in the night than in the 

 day, provided that the growth is not arrested at night by too low a temperature. 



The efl"ect of darkness upon the growth of plants is, hoAvever, 

 differently manifested according to its duration, whether it be con- 



