CHAPTER XIII. 



LACK OF LIGHT. 



Etiolation. 



The disease, which is produced by deficient illumination, or entirle lack 

 of light, is called etiolation (etiolement). The different stem members in 

 the majority of green plants become uncommonly long and weak. Accord- 

 ing to the variety to which they belong, the leaves, as well as the internodes 

 of the stem, either become very long, slender and limp (the majority of 

 monocotyledons), or develop only very slightly and remain, for their whole 

 life, in a condition similar to that in the bud (most dicotyledons). 



A bleaching of the green parts of the plants, i. e., an arrested develop- 

 ment, or decay of the existing chloroplasts, is connected with this change 

 in form. We find exceptions only in the gymnosperms, of which the major- 

 ity are unusually little susceptible to the removal of light. At any rate, 

 according to Burgerstein^ the absorption of the endosperm becomes slower, 

 the epinastic spread of the cotyledons less energetic and incomplete than in 

 the light,' but — with the exception of Gingko hiloha and Ephedra — the seed- 

 lings did turn green, Cycas and Zamia, on the other hand, cannot form 

 any chlorophyll in complete darkness, even with a favorable temperature. 

 Among conifers, the larches need the light most since they become only 

 slightly green when it is excluded, while the Cupressineae become com- 

 pletely green. 



The difference in the formation of the leaves of etiolated plants is 

 explained by the fact that the leaf, for the most part, must nourish itself 

 and that the cellulose material, which it needs for the new formation and 

 maturing of the leaf cells, can be formed only by the action of the light on 

 the very spot. If the nutriment is suppressed, the leaf cells, already formed 

 in the bud, elongate with the absorption of water, on which account the 

 leaf itself will become somewhat larger, but all further growth, depending 

 on cell increase, will be impossible. The more the leaf, in its later enlarge- 

 ment in the Hght, depends on cell increase, the smaller it remains when the 



1 Burg-erstein, A., t)ber das Verhalten der Gj^mnosp'^rmen-Keimlinge im Lichte 

 imd im Dunkeln. Just's bot. Jahresb. 1900, II, p. 250. 



