ACTION OF LIGHT ON VEGETATION. (^n r 



very striking are exiiibited by plants grown in the dark, and constitute the bleached 

 or etiolated condition. The internodes of etiolated plants are in general much 

 longer than those of plants of normal growth; and the long narrow leaves of 

 Monocotyledons are subject to the same change. On the other hand the leaves 

 of Dicotyledons and Ferns usually (but not always) remain very small and do not 

 completely outgrow their bud-condition, or exhibit peculiar abnormalities in their 

 expansion. These peculiarities will be explained more in detail in Chap. IV. It is 

 not necessary however to contrast etiolated plants with those of the normal green 

 colour, in order to establish the influence of light on their growth. If plants of the 

 same species are compared when grown in more or less deep shade with others 

 grown in full daylight, these differences are still more conspicuous, varying ac- 

 cording .to the intensity of the light. Different species are however affected to 

 a different extent by etiolation ; the internodes of climbing plants, which are very 

 long even under normal conditions, become much longer still in the dark; anil 

 some leaves of Dicotyledons, as for instance those of the beet, become tolcrabJv 

 large under the same circumstances, while on the other hand the abnormal! \ 

 elongated internodes of etiolated potato plants put out leaves of only a very small 

 size. It is remarkable that etiolation, as I have already shown ^, does not extend to 

 the flowers^. As long as sufficient quantities of assimilated material have been 

 previously accumulated, or are produced by green leaves exposed to the light, flowers 

 are developed even in continuous deep darkness which are of normal size, form, 

 and colour, with perfect pollen and fertile ovules, ripening their fruits and producing 

 seeds capable of germination. The calyx however, w^hich is ordinarily green, 

 remains yellow or colourless. In order to observe this it is only necessary to 

 allow tulip-bulbs, the rhizomes of Iris, or the like planted in a pot, to put up shoots 

 in complete darkness, when perfectly normal flowers are obtained with completely 

 etiolated leaves. Or a growing bud on a stem of Cucurbita, Tropaeolum, Ipomcea, 

 &c., with several leaves, is made to pass through a small hole into a dark box, 

 the leaves which remain outside being exposed to as strong light as possible. 

 The bud developes in the dark a long colourless shoot with small yellow leaves and 

 a number of flowers, which, except in the colour of the calyx, are in every respect 

 normal ^. The extremely singular appearance of these abnormal shoots with normal 

 flowers shows in a striking manner the difference in the influence of light on the 

 growth of different organs of the same plant. 



The retarding effect of light on the growth of the shoot is evident even in 

 a short time ; and, as I have already briefly shown ^ a periodical oscillation in the 

 rapidity of growth is caused by the alternation of day and night (when the tem- 

 perature is nearly constant). This variation is shown by the growing internode 



^ Sachs, in Bot. Zeitg. 1863, Supplement; and 1865, p. 117. 



2 [An exception to this rule is afforded by the coloured kinds of lilac which are forced during 

 the months of February and March by the market-gardeners of Paris, at a temperature of from 



'33° to 35° C, and in almost complete darkness. The flowers expanded under these conditions are 

 completely white. See Duchartre, Journ. de la Soc Imp. et cent, d'hort. de France, i860, pp. 272- 

 280.— Ed.] 



3 Sometimes however abnormal flowers appear in the dark as well as the normal ones. See 

 Sachs, Exp.-Phys. p. 35. 



* Sachs in Heft II of the Arbeiten des Bot. Inst, in Wiirzburg, 1S72. 



X X 2 



