FLOWER-FORMING RESERVE MATERIALS. 535 



humus-plants, pass through all their essential processes of development, and 

 especially all their processes of embryonic growth, in profound subterranean dark- 

 ness, so that the foliage-leaves only come forth annually and for a short time into the 

 light, in order to assimilate and to fill the subterranean parts with nutritive substances--- 

 and the flowers come forth beyond the surface of the earth not because they require 

 light for their growth, but in order to ensure pollination by the aid of insects. 



I have so far, for the sake of simplicity, in considering the subject of etiolation, 

 always regarded only the contrast between full daylight and profound darkness ; it 

 must be added, however, that etiolation also occurs in feeble light, and is the more 

 marked in proportion to the diminution in intensity of the light. Here, however, it 

 is a matter of one important point chiefly, that is whether the intensity of the light is 

 at all sufiBcient to effect assimilation in the chlorophyll ; if this is not the case, as for 

 example often happens in the interior of a dwelling-room, then though green leaves 

 may be formed, it is true, so long as the store of nutriment lasts, still, since no 

 new assimilation takes place, this store is gradually exhausted, and the plants grow 

 themselves to death — a very common case when they are left on flower -stands 

 in rooms. 



In contrast to the unusually energetic growth which takes place even in profound 

 darkness when there is a considerable store of assimilated substance accumulated, 

 either in reservoirs of reserve-materials (seeds, bulbs, tubers), or when it is produced 

 at the time by assimilating green leaves, are now to be mentioned the spores and 

 gemmae of the Cryptogams, which so far as is known do not germinate at all in the 

 dark, although in most cases a feeble light is sufficient to induce their growth. 



I must content myself with these few remarks with reference to the questions 

 connected with etiolation, although much more might be added. 



I will now quote a few more cases belonging to the above-named second 

 category, concerning the fact that organs which normally develope in the dark may 

 be abnormally compelled to grow in daylight. Here again the common Potato, as 

 in so many other cases, forms an unusually favourable object for physiological 

 observations. It may be premised in the first place that potato-tubers lying in a dark 

 damp place develope, as is well known, numerous etiolated shoots, which are often 

 very long, and which are furnished with extremely small leaves : in this, after what 

 has already been said, there is simply nothing peculiar. However, if potato-tubers 

 are laid on damp sand, for instance, and covered with a transparent bell-jar in the 

 spring and summer and kept at a bright window, there grow out it is true numerous 

 thin roots from the so-called ' eyes ' (buds) but the shoots themselves remain for 2-3 

 months of continued cultivation extremely short, and none of their leaves are 

 developed — ^in other words, they are extremely sensitive to the action of light, which 

 retards their growth to a striking extent. So far as I understand this phenomenon 

 (which still needs careful investigation), it chiefly depends simply upon the fact that 

 the potato-tuber in the normal course of life is covered with several centimetres of 

 soil, and must therefore also be darkened when the sprouting shoots develope from 

 the eyes. It is chiefly the growth of the 2-3 lowermost internodes of the shoot 

 which are concerned here, and it is from these that the roots and stolons arise. 

 These lowermost segments of the shoot are adapted to darkness and can scarcely 

 grow in the light; whereas the later segments and rudiments of leaves, on the 



