«KCT. II 



PHYSIOLOGY 



263 



tiiiuous or interrupted, ;is in the changes of night iind day. 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 greatly retarded. 

 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. 

 212, E). A plant grown under such conditions 

 is spoken of as ETIOLATED (''^). 



The elongation of certain organs and simultaneous 

 reduction of otliers has an oecological significance in 

 nature in the case of seedlings and rhizomes which are 

 growing in the dark. The parts which are functional 

 only in the light remain at first undeveloped, and the 

 construction material for them and especially for the 

 chlorophyll is economised. The great elongation of the 

 other organs which is mainly dependent on an accumula- 

 tion of Avatei", brings the parts that need it as soon as 

 possible into light. 



Etiolation does not occur in all plants as in the 

 typical Dicotyledon. The stems, for instance, of certain 

 Cacti are much shorter when grown in the dark tlian in 

 the light, and tlieir flattened shoots remain cylindrical. 

 Similarly, the leaves of varieties of the Beet {Beta) grow 

 as large, or even larger, in the dark than in the light ; 

 this is also true, under conditions favourable to nutrition, 

 of the leaves of other plants. In some Monocotyledons, 

 narrow but very long leaves are developed. 



Just as the rays of light of different wave- 

 length were found to be of different value in the 

 process of assimilation, so growth is by no 

 means equal in differently coloured light. It 

 is to the strongly refractive so-called chemical Fig. 212.— Two seedlings of 

 rays that the influence of light on growth is S"iap« ciZ6a of equal age ; 



1 ,, 1 11 T (• 1 ^i grown in the dark, 



due ; the red-yellow end or the spectrum acts etiolated ; n, grown in or- 



upon many plants in the same manner as 



darkness. 



Radium and the Rontgen rays tend to arrest the processes of 

 growth and development i^^). 



Since elongation is essentially due to the introduction of water, 

 the significance of the water supply to a growing plant is obvious. 

 Growth often ceases when there is not sufficient water in the soil. 

 Even a diminution in the moistness of the air may arrest growth by 

 increasing transpiration.^* Some plants, however, can store water, and 

 are therefore more independent of its direct absorption. They grow 



dinary daylight, normal. 

 The loots bear root-liairs. 



