DEPENDENCE OF PLANTS ON HEAT, LIGHT, ETC. 1 97 



There is, in the first place, the striking contrast of the winter rest of vegetation, 

 as opposed to the vitality which the unfolding of the shoots and flowers in 

 spring and summer presents. In the main it is the lower temperature of winter 

 which renders the vital phenomena of the plant impossible ; only when the 

 temperature of the air and of the earth rises several degrees above the freezing 

 poinr of water, with a higher position of the sun, do the buds of trees, subterranean 

 rhizomes, bulbs, and tubers begin to be active and grow. At first, this is hardly per- 

 ceptible, and very slow ; but with the appearance of the first warm spring days, the 

 shoots come forth into the light with striking rapidity, and in a few days the 

 whole aspect of nature is changed. The surface of the earth is brilliant with 

 vivid green, flowers make their appearance, and in a few weeks one hardly 

 remembers how bare and lifeless the winter landscape was. The contrast appears 

 no less conspicuous when, coming from the hot air of the plains, in the middle of 

 summer, we ascend into the cool climate of Alpine heights, or journey towards 

 the far north ; we then leave a fully developed vegetation, to go towards a delayed 

 spring. When our trees and fields are preparing for the autumn, vegetable life 

 is just beginning on the mountains, and in high northern latitudes. If closely allied 

 forms of plants are compared, of which the one is native with us and the others are 

 at home between the Tropics, we very often find the former small, succulent 

 herbs and shrubs, while the latter develope into woody bushes or huge trees — a 

 difference which depends chiefly upon the fact that our summer is cool and short, 

 while that of the Tropics is hot and long. Any one, finally, who has to cultivate 

 plants of difl"erent climates on the same spot, e. g. in a botanical garden, knows 

 with what great cost and trouble this is done. The construction of greenhouses 

 and the costly labour are chiefly, if not exclusively, caused by the different require- 

 ments -of warmth of plants coming from different climates. 



Still more various, if possible, are the phenomena due to the dependence of 

 the life of the plant upon Light ^- 



If, for example, we lead the terminal bud of a vigorous leafy shoot into the dark 

 cavity of a box with opaque walls, a system of shoots, leaves, flowers, roots, tendrils, 

 and even ripe fruits with seeds capable of germinating, develope on it. The whole, 

 however, presents an exceedingly strange appearance ; the shoot-axes and leaf-stalks 

 are quite white ; the laminae of the leaves are small, and coloured yellow instead of 

 green, and generally are not extended flat ; and the entire substance of these so-called 

 etiolated parts is richer in water, more delicate, and more sensitive to injurious 

 influences than normal ones developed in the light. The flowers developed in the 

 dark, however, attain their full beauty of colour and size. If now a bud of this 

 shoot-system, grown in the dark, is conducted through another narrow opening of 

 the box out into the light again, normal, green, flat-extended and large leaves 

 are produced once more ; and the shoot, growing forward in the light, again obtains 

 its normal peculiarities in every respect. This experiment, as simple as it is in- 

 structive, shows that the growth of all the organs can take place, even in deep 



' I published the first detailed investigation on the so-called etiolation of plants in my treatise 

 ' Ueber den Einfluss des Tageslichts auf Neubildung und Enfaltung vtrschiedener Pflanzenorgane,' 

 in Bot. Zeit. 1863, and further in the treatise, ' Wirkung des Lichts auf die Bliithenbildung unter 

 Vermittlung der Laubbldtter,' Bot. Zeit., p. 117. 



