RESPIRATION. 493 



to roots and tubers, stems and foliage, fruits and seeds, green plants and parasites 

 devoid of chlorophyll, plants with and without stomata, saprophytes and water 

 plants. All these breathe as long as they are alive, and in plants no less than 

 in animals breathing and living can be used as synonymous terms for all practical 

 purposes. The first fundamental condition of respiration is naturally the presence 

 of free atmospheric oxygen. When this is absent, plants, like animals, are suffo- 

 cated and die. If plants are put under the receiver of an air-pump, from which 

 the air is exhausted, or in a chamber filled with hydrogen, nitrogen, or coal-gas, 

 the streaming movement of the protoplasm in the cells ceases in a short time, 

 foliage and floral-leaves, if they exhibit phenomena of movement in the living 

 plants, become rigid, and if kept for a longer period in the atmosphere without 

 oxygen, the plants die. Even if again brought into air containing oxygen, they 

 can no longer be resuscitated, but remain dead. 



The parts of plants surrounded by atmospheric air are never in want of 

 oxygen, but roots often get into an unfavourable position where the quantity of 

 oxygen in the air of the soil is very small, or where the atmospheric air is replaced 

 by other gases. This explains why plants do not prosper in so-called " dead " 

 earth, and why the roots seek principally those loose places of the upper strata 

 of soil which are porous and well-ventilated, and avoid the deeper-lying, badly- 

 ventilated, dead ground. The decay of trees which have been planted in towns 

 and parks near gas-pipes, whose roots have been surrounded with coal-gas for some 

 time owing to a leak in the pipes, is also explicable in this way. 



Aquatic plants take up the oxygen of the atmospheric air absorbed by the 

 water. Where there is none, vegetable life under water becomes impossible. If 

 anyone, when sending off" water plants, tightly corks up the bottle after filling 

 with the necessary water, under the impression that the plants, being still in their 

 element, will thus bear the journey well, he will be sadly undeceived. The small 

 quantity of oxygen in the atmospheric air contained by the water is soon 

 exhausted, and the aquatic plants are sufibcated within twenty-four hours, or even 

 in a much shorter time, just like fishes which have been conveyed in a tightly- 

 corked bottle of water. 



All plants do not breathe with the same energy, and in any plant a great 

 difference can be noticed in the respiration of the various organs. The floral- 

 leaves, possessing no chlorophyll, respire much more vigorously than the green 

 foliage-leaves; underground root-stocks, bulbs, and tubers, also without chlorophyll, 

 to a much greater extent than the green stem. In the green organs of plants 

 exposed to sunlight two processes are carried on, the formation and the splitting 

 up of carbohydrates. The latter process, however, is so obscured by the former, 

 that it can only be observed with diflSculty. It has been estimated that in a 

 laurel leaf the amount of carbohydrates formed in any given time is thirty times 

 as great as of those decomposed, i.e. respired. 



A great difference is also exhibited according to the stage of development of the 

 individual plant organs. Roots^ stems, and leaves breathe much more vigorously 



