y^S GENERAL CONDITIONS OF PLANT-LIFE. 



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

 lorhiza, Monotropa^ Lathrcea, Orobanche, &c. Even many plants which do contain 

 chlorophyll 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, TropcBolum, IpomcBa, 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, which 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 require detailed consideration, 

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

 afl"ect vegetation in a manner altogether diff"erent; certain functions are induced 

 only by the highly refrangible rays, others only or chiefly by those of lower refran- 

 gibility. These efl"ects moreover vary not only with the temperature but also with 

 the intensity of the particular rays. Finally it must be observed that light afl"ects 

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

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

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

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



(i) Action of rays of different refrangibility. The rays of different 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 or sugar. 



On the other hand the rays of high refrangibility (the blue or violet, as well 

 as the invisible ultra-violet rays) are the principal or the only ones which produce 

 mechanical changes so far as these are dependent on light. It is these rays which 

 influence the rapidity of growth, alter the movements of the protoplasm, compel 

 swarm-spores to adopt a definite direction in their motion, and change the tension 

 of the tissues of the motile organs of many leaves and hence affect their position. 



