PHYSIOLOGY 



259 



varieties of the Beet (Beta) grow as large, or even larger, in tin- dark than in the 

 light ; this is also true, under conditions favourable to nutrition, of the leaves of 

 other plants (Cucurbita). In the shade of a forest leaves often become larger than 

 in full daylight. They are then proportionally thinner, and the palisade cells 

 which, in leaves fully exposed to the light, are in close contact, become pointed 

 below, and thus leave large intercellular spaces between 

 them. In this way the modifying influence of light of 

 diminished intensity is apparent in the internal structure 

 of such shade : leaves. Flowers, however, if sufficient 

 constructive material be provided by the assimilating 

 leaves, develop, according to SACHS' observations, as well 

 in the dark as in the sunlight, except that they are some- 

 times paler in colour. If, however, the assimilatory 

 activity of the green leaves be reduced or destroyed by 

 depriving them of light, many plants, as VOCHTING 

 found, form only inconspicuous or cleistogamous flowers ; 

 as (JOKBEL shows, the same result may follow when the 

 nutrition of the flowers is interfered with by poor soil or 

 the excessive development of the vegetative organs. 



The tissues of etiolated stems and leaf-stalks are richer 

 in water and thinner- walled than in normally growing 

 plants. Even the roots of such plants are often found 

 to be less strongly developed. The supply of reserve 

 material at the disposal of plants growing in the dark 

 is utilised, with the assistance of an unusual amount 

 of distension water, in the elongation of the axis. This 

 elongation of the shoot axis, resulting from growth in 

 darkness, is of especial value in the development of 

 young plants from underground tubers, rhizomes, and 

 seeds. When the leaves must themselves reach the light 

 by their own elongation, as in many Monocotyledons, 

 they act just as do the stems of Dicotyledons, and attain 

 an abnormal length in the dark. 



From what has already been said it would 

 seem that plants must grow more rapidly during 

 the night than day, and this is actually the case 

 where other conditions affecting growth remain Fl - - >18 -~ Two s edlin s s 

 the same by night as by day. A too low 

 temperature during the night may, however, 

 completely counteract the accelerating influence 

 of darkness upon the growth. 



.Just as the rays of light of different wave-length and refrangibility 

 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 RAYS THAT THE 



INFLUENCE OF LIGHT ON GROWTH is DUE; the red-yellow end of the 

 spectrum acts upon many plants in the same manner as darkness. 



Kadium and the Kontgen rays tend according to M. KORNICKE to 

 arrest the processes of growth and development ( 71 ). 



of 



Sinapis alba, of equal age ; 

 K, grown in the dark, 

 etiolated ; N, grown in or- 

 dinary daylight, normal. 

 The roots bear root-liaiis. 



