494 



RESPIRATION. 



when young than when fully formed. When seeds are allowed to germinate in 

 damp earth, respiration is at first quite inconsiderable, but when the parts of 

 the seedling begin to elongate, and when the stores of materials furnished by 

 the parent plant are dissolved and used up, respiration becomes very energetic. 

 Later on, when the seedling has grown up so far that it can itself work with 

 the help of its leaves, which have meanwhile become green, respiration again 

 diminishes. The same thing occurs in the development of buds; there, too, the 

 young leaves just emerging from the covering of the bud breathe to a much 

 greater extent than the fully-formed green foliage. That organs which have 

 attained their full size, and are apparently quite inactive, still respire, is shown 

 from the observation that roots and tubers which have been dug up in the 

 autumn and left in a cellar through the winter exhale carbonic acid without 

 any visible outward change. In beet-roots which have been dug up a 1-per-cent 

 decrease of sugar, and an exhalation of carbonic acid corresponding to this decrease, 

 have been observed within two months, a proof that change of materials and 

 respiration can occur even in structures which lie dormant during the winter. 



According to what has just been stated about the significance of respiration 

 to the life of plants, it is quite obvious that the energy of respiration which is 

 reckoned by the amount of carbonic acid exhaled from a certain organized mass, 

 or better, by the amount of oxygen absorbed, becomes greater the more vigorously 

 the plant grows, and the quicker it builds up its body, just as a machine requires 

 more fuel the greater the results required from it. If fuel is wanting or not 

 present in sufficient quantity the machine stops, or does not perform as much 

 work as it should be capable of doing. It is exactly the same in living plants. 

 If the respiratory materials are absent, respiration is discontinued even in the 

 presence of oxygen, and life becomes extinguished. If the supply of these 

 materials is insufficient the plants only prolong their existence with difficulty, 

 and their growth will be insignificant in consequence of the insufficiency of 

 materials for carrying on the work. When shoots sprout from the "eyes" of a 

 potato, it is at the cost of the carbohydrates and other materials stored up in 

 the tuber. If this sprouting occurs in the open and the shoots grow up into 

 the daylight, their leaves become green and manufacture new carbohydrates 

 under the influence of the sun's rays; and of these a portion is at once employed 

 as fuel for the further construction of the potato plants, that is to say, it is 

 respired. If, on the other hand, the development of shoots from potatoes takes 

 place in a dark cellar, their leaves cannot become green, and consequently no 

 carbohydrates can be manufactured. The shoots then only continue to grow so 

 long as the respiratory materials stored up in the tuber last; when these are 

 exhausted, respiration comes to an end, and the shoots die off". 



An approximate idea of the significance of respiration as an impelling force 

 in those changes of materials whose end is the further growth of the plant may 

 be obtained from a consideration of the following figures. A cubic centimetre 

 of carbon dioxide contains 0'5376 milligramme of carbon, whose burning furnishes 



