258 BOTANY PAIST i 



cessation of growth at a temperature less than or higher than 

 40-50. Between the MINIMUM and MAXIM CM temperatures, at which 

 growth ceases, there lies an OPTIMUM temperature (p. 176), 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 include a wide range, as they vary for 

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

 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. In 

 like manner, the optimum and maximum temperatures show great 

 variation in different species of plants. Thus some Algae and Bacteria 

 grow in hot springs at a temperature of 80 C., which would be 

 at once fatal to other plants. The optimum does not usually lie in 

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

 maximum. (On the dependence of the cardinal points on external 

 conditions cf. p. 176.) 



A certain amount of variation in the temperature favours the germination of 

 seeds and the unfolding of young shoots more than exposure to any constant 

 temperature. This is due to the optima for the different processes concerned lying 

 at different temperatures ( 70 ). 



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 white in colour. The leaf- 

 blades are small and of a bright yellow colour, and remain for a long 

 time folded in the bud (Fig. 218, E). A plant grown under such con- 

 ditions is spoken of as " etiolated."- 



This diminution in the size of the leaf-blades and the elongation of the stem 

 (and leaf-stalks) are not manifested by all plants, nor under all circumstances. The 

 stems, for instance, of certain Cacti are much shorter when grown in the dark than 

 in the light, and their flattened shoots remain cylindrical. Similarly, the leaves of 



