ETIOLATION. 53 1 



developed in the dark become yellow, it is true, whereby of course a step towards 

 the formation of chlorophyll is accomplished, but the greening of this yellow imperfect 

 chlorophyll is only completed when such etiolated leaves are subsequently exposed to 

 light. Moreover, the size of a plant thus developed in the dark, as well as the number of 

 roots and leaves formed, are proportional chiefly to the volume and mass of the seed 

 or, better, to the quantity of the reserve-materials accumulated in it. From the minute 

 seed of the Tobacco plant there developes in the dark a correspondingly tiny seedling 

 with two cotyledons ; from the large seed of a Bean or of a Horse Chestnut, on the con- 

 trary, there may be produced in the dark a plant of considerable size with copiously 

 branched roots, and several, though small and yellow, leaves. At length, however, after 

 a few days, or in the case of very large seeds after a few weeks, the growth in the dark 

 always ceases, and the whole plant becomes diseased and perishes ; in many cases, as 

 WiihFhaseolus, &c., not until all the reserve-materials of the seed have been completely 

 used up for the formation of organs ; in other cases, however, as in the Gourd, growth 

 and the development of organs cease while the cotyledons still contain considerable 

 quantities of unused formative substance. The malformation of such etiolated 

 seedlings, as well as their final perishing, show that they are in a sickly condition, 

 and this at length affects the processes of growth proper, which alone interest us here. 

 Although the fact that the shoot-axes become elongated more rapidly in the dark 

 agrees with the observation that their growth under normal conditions of illumination 

 takes place more rapidly during the night, or during occasional short periods of 

 darkness, than under the influence of light — a point to which I shall return in 

 the next lecture ; yet the dwarfing of the etiolated leaves of seedlings might lead 

 to the conclusion that their growth is directly retarded or inhibited by deficient 

 light, and thus might be accelerated by illumination. That is a mistake, however, 

 since, as we shall see later, healthy green foUage-leaves which have previously been 

 exposed for some time to the action of light, grow more rapidly during the night or 

 during other periods of darkness which last for a few hours only. From this it 

 evidently follows that the dwarfing of etiolated leaves of seedlings must have other 

 causes, which \ve simply imply by terming the etiolated leaves (and the same is true 

 of the excessively long shoot-axes) diseased. Of course this appears to be con- 

 tradicted by the fact that young etiolated leaves when brought into the light grow 

 vigorously after a few days ; but this growth is not a direct result of the action of 

 light, but rather a consequence of the fact that, by means of the illumination, in the 

 first place a normal healthy condition is created in the tissue, and this in its turn 

 makes further growth possible. 



That seedlings which have grown up in the dark soon fall victims to an actual 

 disease of their tissues, and that their abnormal relations of growth depend essentially 

 on morbid changes in their tissues, due to want of light, and that consequently points 

 come under consideration which have hitherto scarcely been noticed, and that, above 

 all, the supply of suitable constructive materials plays a great part, it is easy to 

 convince ourselves by a modification of the experiment. 



Instead of allowing seedlings to grow up from seed in the dark, I directed 



leaves developing in the dark from the older stem of various Ferns form chlorophyll, which, on the 

 contrary, is not the case with Equisdum. 



M m 2 



