mv. it 



PHYSIOLOGY 



temperature usually lies between 22 and 37 C. Plants inhabiting 

 different climates exhibit considerable differences in regard to the 

 cardinal points for temperature (cf. p. 219). 

 That the different individuals of the same 

 species may show great differences in the 

 dependence of the phase of elongation on tem- 

 perature is seen in the unequal development of 1L I 

 the buds of the Horse-chestnut, etc., in spring. 

 Even in the same individual the processes of 

 growth in the different organs are variously 

 influenced by the temperature. 



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 penetrate 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 tempera- 

 ture than the unfolding of the foliage leaves. 



2. Light. -The growth of a plant is rarely 

 so strictly limited to a particular illumination as 

 to a particular temperature. There are, however, 

 some organs in which growth commences only 

 after a certain intensity of light has been experi- 

 enced; some seeds (p. 305) and all parts of plants 

 which are normally exposed to light can only 

 continue their development when this is present. 

 Long-continued darkness produces an abnormal 

 growth, in that the normal correlation between 

 different organs is disturbed ; the growth of 



certain organs is unduly favoured, and of others FIG. 260. TWO seedlings of 

 ^ i 11 ' -" "" * ' 



greatly retarded. In darkness the yellow pig- 

 ment of the chloroplasts but not the chlorophyll 

 is formed. The stems of Dicotyledons, in such 

 cases, 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. 260 E). A plant grown under such conditions is spoken of as 



ETIOLATED. 



The elongation of certain organs and simultaneotts reduction of others has 

 an ecological significance in nature in the case of seedlings and rhizomes which 

 are growing in the dai'k. The parts which are functional only in the light remain 

 at first undeveloped, and the constructive material for them and especially for 

 the chlorophyll is economised. The great elongation of the other organs which 

 is mainly dependent on an accumulation of water brings the parts that need it 

 as soon as possible into the light. 



U 



Sinapis alba of equal age. 

 E, Grown in the dark, 

 etiolated ; N, grown in 

 ordinary daylight, normal. 

 The roots bear root-hairs. 

 (After XOLL.) 



