PHOTOMORPH1C ACTIONS 



89 



when a shoot is led into a dark chamber, only the part in darkness is etiolated, and 

 if the distal extremity grows into light again, it assumes the normal shape. Under 

 such circumstances starvation is avoided, and hence the leaves on the darkened 

 portion of the stem are usually larger than those formed on plants kept in total 

 darkness 1 . On the other hand, the action of light in 

 favouring the development of the illuminated parts 

 may, by correlation, retard the growth of the darkened 

 portions, and in fact Jost 2 obtained non-green leaves 

 of normal size and shape on darkened shoots of 

 Mimosa pitdica and Phaseolus niultiflorus by con- 

 tinually removing the buds formed on the parts ex- 

 posed to light. Jost 3 also observed that the buds 

 on an attached branch of the beech did not develop 

 in darkness, but did so when the branch was isolated. 

 Similarly, Godlewski 4 found that an increased activity 

 of growth in the hypocotyls of seedlings of Raphanus 

 sativus retarded the growth of the cotyledons, and 

 vice versa. Since such correlating actions always 

 enter into play, it is not possible to say whether 

 the greater production of runners in Glechoma hederacea, 

 and of climbing shoots in Ampdopsis hederacea^, in 

 weak light, is due to the direct action of the weak 

 illumination upon these organs, or to the retardation 

 of the growth of the main shoot. It is also a pheno- 

 menon of correlation when the removal of the foliage 

 causes the primordia of bud-scales to develop into 



foliage leaves. The scales on the rhizomes of Adoxa, Paris, &c., are, however, 

 unable to develop into green leaves when the primordia are exposed to light, 

 whereas on the stolons of Hicracium* and Circaea" the primordia develop into 

 foliage leaves when illuminated, and into scale-leaves in darkness. 



In the case of fungi, and other heterotrophic plants, the etiolation 

 effects produced by the absence of light are not complicated by any 

 direct influence upon nutrition. Even in the case of autrophic plants 

 the etiolation effects are mainly the result of the absence of the 

 stimulating action of light. Certain fungi, algae, and mosses are in fact 

 unable, in the absence of light, to progress beyond a certain embryonic 

 stage, and are unable to produce reproductive organs. 



FIG. 2.5. Apparatus for partial 

 etiolation. The apex of the flower- 

 ing shoot b is passed through the 

 opening k in the clay saucer t, 

 which is closed by cork and cotton- 

 wool. The dark cylinder ^ is 

 pressed into the sand covering the 

 floor of/. 



1 Sachs, Vorlesungen, 1887, 2. Aufl., p. 541 ; Amelung, Flora, 1894, p. 204; Teodoresco, 

 Rev. gen. d. Bot., 1889, T. XI, p. 369. 



2 Jost, Jahrb. f. wiss. Bot, 1897, Bd. xxvil, p. 478 ; 1898, Ed. xxxi, p. 377. 



3 Id., Ber. d. Bot. Ges., 1894, p. 194. 



4 Godlewski, Bot. Ztg., 1879, p. 105. 



5 Maige, Compt. rend., 1898, T. cxxvn, p. 420. 



6 Nageli, Sitzungsb. d. Munch. Akad., 1866, II, p. 209. 



7 Goebel, Bot. Ztg., iSSo, p. 794; Organography, 1900, I, p. 256. 



