^JO LECTURE XXXI. 



we simply perceive the cause and the last effect, while the probably very long series 

 of intermediate causes and effects remain for the time being unknown. The 

 same is the case, indeed, with all the phenomena of irritability, among which even 

 the best-known, both in animal and vegetable organisms, always present ultimately 

 something entirely inexplicable and unintelligible according to ordinary ideas of 

 mechanics. 



Here again our information as to the influence of light on the development of 

 organs, the rudiments of which are already formed, is much more abundant, although 

 here also, as in the whole of this province, we have as yet gained only the first dim 

 glimpses into very involved processes of Nature. We may in the first place keep 

 apart two cases ; on the one hand we ask the question, what happens when organs 

 which normally complete their post-embryonic growth under the influence of light are 

 necessitated to complete their development in constant darkness ? We have in this 

 case to do with the so-called Etiolation of plants. In the second place, the case is 

 to be noticed where organs, especially leaf-forming shoots, which, as a rule, pass 

 through their whole course of development underground, and therefore in profound 

 darkness, are compelled to do this under the influence of light. 



In spite of the very numerous researches on Etiolation, as it is called — i. e. the 

 alteration induced by want of light in organs which normally grow in strong light- 

 it must nevertheless unfortunately be confessed that we here enter upon a province 

 which until quite recently has been closely bestrewn with gross errors. However, 

 this is not the place to discuss that matter, and I must be satisfied with acquainting 

 the reader with those facts which can easily be observed ^. 



If the seeds of Phanerogams are sown in soil and allowed to develope their 

 seedlings in the dark, there appear after a short time abnormalities, in general of 

 such a kind that the shoot-axes are longer by far than is the case when the 

 illumination is normal, whereas the first leaves of the seedling usually remain much 

 smaller, and do not complete their extension in one plane, and at the same time 

 (except in the Coniferse ^) the formation of chlorophyll is suppressed : the leaves 



' What is here said concerning etiolation is based on my two treatises, ' ffier den Einflitss des 

 Tageslichtes auf NeuHldung und Entfaltung verschiedener Pflanunorgane! Bot. Zeitg. 1863, 

 supplement ; and 1865, Wirkung des Lichtes auf die Bliithenbildung unter Vermittlung der Laub- 

 bldtter, pp. 117, &c. I have of late years carried on in Wiirzburg similar investigations with the aid 

 of better methods than were to hand at that time : the results of these have not yet been published 

 in detail. Gregor Kraus (' ffber die Ursachen der Formdnderungen etiolirier Pflanxen^ Jahrb. f iir 

 Wiss. Bot. VII, p. 209) and Godlewski {'Ziir Kenntniss der Ursachen der Formdnderungen etio- 

 lirier Pflanzen' Bot. Zeitg. 1879, pp. 81, &c.) have both dealt with the excessive lengthening 

 of the internodes and the dwarfing of the leaves of seedlings in the dark, but without taking 

 into consideration my much older statements on the etiolation of shoots nourished by foliage- 

 leaves exposed to light. Moreover, the text above is chiefly concerned not with the mechanical causes, 

 but only with the facts themselves, wherefore 1 need not here go further into the statements of these 

 observers. Kraus' assumption, in particular, that etiolated leaves remain small because leaves in 

 general are only able to grow from their own products of assimilation, is strikingly contradicted 

 by the experiments described and illustrated in the text. That the dwarfing of leaves in the 

 dark is a phenomenon of disease, and cannot be compared with the growth of leaves under 

 normal conditions, has been shown by Prantl's investigations in my laboratory, ' Ober den Einfluss 

 des Lichtes auf das Wachsthum der Blatter' (Arb. des bot. Inst, in Wzbg., B. I, p. 371). 



" That the seedlings of Conifers produce leaves which contain chlorophyll even in profound 

 darkness I first stated in the publication 'Lotos' (Prague, Jan. 1859); ^°^ ■'■ showed later that 



