6f:0 GENERAL CONDITIONS OF PLANT-LIFE. 



wise excluded from light are nourished by the products of assimilation produced in 

 the light, so also parasites and saprophytes destitute of chlorophyll live, as has 

 already been explained, on the work performed by plants that contain chlorophyll, 

 and are therefore dependent indirectly on light, even though the whole of their 

 development may be completed in darkness, as in the truffle ; in other instances 

 they only emerge to unfold in the air the flowers already formed underground, and 

 disseminate their seeds, as is the case with Limodorum aboriivum., Epipogium, Coral- 

 lorhiza, IMonotropa, Lathraea, Orobanche, &c. Even many plants which do contain 

 chlorophj'Il and which live on inorganic food complete their growth and the pro- 

 cesses connected with it in complete darkness, only putting forth their green leaves 

 at certain times for the purpose of again accumulating beneath the ground fresh 

 formative material. This is the case with the autumn crocus, tulip, crown imperial, 

 terrestrial orchids, and many others, and especially with plants which form bulbs, 

 tubers, and rhizomes. If the growing end of a stem of a green-leaved plant {e. g. 

 Cucurbita, Tropeeolum, Ipomoea, or Hedera) is secluded from all light while the 

 green leaves remain exposed to it, the buds develope in the dark ; leaves and flowers 

 are produced, M'hich latter attain their full size and beauty of colour, are capable 

 of fertilisation, and produce fruits and even fertile seeds at the expense of the 

 substance assimilated in the light in the green leaves and carried to them by 

 the stem. 



These and a number of other facts show that growth, /. e. the processes by 

 which the form of the plant is attained, and metastasis are not necessarily dependent, 

 or only to a subordinate extent, on the influence of light, if only the necessary 

 quantity of assimilated material has previously been accumulated. 



This is a general statement of the case. If however the various separate 

 processes of vegetation are observed — the behaviour of protoplasm, the formation, 

 arrangement, activity, and destruction of chlorophyll, the growth of the younger 

 and older parts, the movements resulting from the tension of the tissues, &c. — a 

 long series of very varied facts presents itself which requires detailed consideration, 

 because the rays of different refrangibility which are mingled in white daylight 

 affect vegetation in a manner altogether different ; certain functions are induced 

 only by the strongly refrangible rays, others only or chiefly by those of less refran- 

 gibility. These effects moreover vary not only with the temperature but also with 

 the intensity of the particular rays. Finally it must be observed that light affects 

 plants only when its rays penetrate into their organs ; this however modifies them 

 in intensity and to a certain extent also in refrangibility. In every investigation of 

 the action of light these points must therefore be kept in view. The following 

 summarises what is at present knowm as to the general facts. 



(i) Action of 1- ays of different refrajigibility. The rays of difl"erent refrangibility 

 commingled in white sunlight which appear as variously coloured bands in the 

 spectrum, vary in their physiological action on the processes of vegetation. Chemical 

 changes, so far as they are in the main dependent on light, are produced chiefly 

 or solely by rays of medium or low refrangibility {viz. the red, orange, yellow, or 

 green). This is the case for instance with the production of the green colour of 

 chlorophyll, the decomposition of carbon dioxide, and the formation in chloro- 

 phyll of starch, sugar, or oil. 



