496 DEVELOPMENT OF LIGHT AND HEAT. 



and live. It has been estimated that a plant, in which the supplies of freshly- 

 formed carbohydrates are lacking, can consume over 50 per cent of its substance 

 by respiration before it perishes from exhaustion. This is the case, for example, 

 in the potato-tubers mentioned, whose stems, developed in dark chambers, become 

 overgrown, i.e. elongate exceedingly, while their rudimentary foliage-leaves remain 

 very small and destitute of chlorophyll. Here, in the dark, no new formation of 

 carbohydrates occurs, but respiration continues as long as any respirable materials 

 yet remain. At length, when everything that can be made use of in this way is 

 consumed, the shoots die off. Their dry weight, however, is only half as much as 

 was that of the tuber from which they sprung; the other half has been completely 

 burnt, i.e. changed into carbonic acid and water, which have rapidly evaporated. 



Sunlight is not necessary to respiration, although without it the decomposition 

 of carbonic acid and the formation of carbohydrates cannot take place. Breathing 

 can be carried on in complete darkness. Underground organs: roots, tubers, 

 bulbs, rhizomes, runners, likewise the mycelia and fruit-stalks of the plants 

 classed together as fungi, as well as seeds buried in the earth all these normally 

 breathe in darkness. Respiration continues throughout the darkest night. That 

 growth, the most important of all the processes stimulated by respiration, is 

 restricted by the influence of light, will be discussed when describing growth; 

 concentrated light produces a rapid oxidation and disorganization of the organ 

 exposed, which, however, must not be looked upon as the respiration of the plant. 



DEVELOPMENT OF LIGHT AND HEAT. 



It is to be expected that respiration will be more vigorous in plants the higher 

 the temperature, since the process of respiration is a combustion of carbon com- 

 pounds, and all combustion is helped by a rise of temperature. As a matter of 

 fact, it has been observed that the exhalation of carbonic acid (that is to say, 

 respiration) also increases with rise of temperature. Of course this is true only 

 up to a certain point. Respiration may commence even at 0, and reaches a 

 maximum between 15 and 35 C. according to the species, but beyond that it 

 quickly diminishes. It entirely ceases at temperatures which produce coagulation 

 of the proteids, and which are followed by the death of the living protoplasm. 



When once respiration is started, the oxygen necessary for the combustion of 

 carbohydrates is derived from the surrounding atmospheric air. But the first 

 incitement to respiration does not proceed from this, or in other words, the 

 absorbed oxygen does not furnish the first stimulus to respiration. Dead plants 

 into which oxygen is made to enter do not breathe any more than do butterflies 

 which have been suffocated by withdrawal of oxygen, and then subsequently 

 brought into the fresh air. Oxygen cannot produce those movements of the 

 atoms which are peculiar to life either in plants or animals which have been 

 completely suffocated. Since only living plants can breathe, respiration must 

 be brought about by a force which is liberated in the living protoplasm, by that 



