146 



in oxygen, exchanging it in the living cells for carbon dioxide, 

 and returning the latter to the surrounding air. 



Even in regard to the amount and rate of respiration the sup- 

 posed difference between animals and plants breaks down. Under 

 favorable conditions the process may be even more active in 

 plants than in animals. In man the carbon dioxide produced 

 in twenty-four hours equals about 1.2 percent, of the body 

 weight, but in some of the moulds the amount has been found to 

 equal 6 per cent, of the dry weight of the plant. Bulk for bulk, 

 very active bacteria may consume oxygen 200 times more rapidly 



ofhea 





ies with the 



age of the organism, and with external conditions. Breathing, 

 which is the expression of respiration in man, is most rapid with 

 infants, and decreases with the approach of old age. So it 

 is with plants, for germinating seeds and young seedlings fespire 

 more rapidly than mature plants. Increase 



vith ir 



ilso, the 



process is more vigorous in the spring, during the work of bud- 

 opening and the putting forth of new leaves and flowers. Under 

 bodily pain or mental excitement we breathe more rapidly, so 

 also does a plant that has been cut, or otherwise injured, or sub- 

 jected to any stimulus, as, for example, violent shaking. A ther- 

 mometer placed in a dish of cut onions, for example, will indicate 

 the existence of a fever (due to the wounding of the tissue), just 

 as surely as if placed in the mouth of a typhoid patient. 



This question is far from having a merely academic interest, 

 Practices that have been in vogue since man first began to till 

 the soil, and that must be continued as long as agriculture is 



plants. I refer to plowing the soil and hoeing the crops. It is 

 not alone to get the soil into a suitable physical condition that it 

 is broken up by the farmer. The roots and other underground 

 parts must have air to respire, just as much as the parts above 

 ground, but if the soil is hard and compact this need is but poorly 

 met. The plow, the spade, and the hoe facilitate the thorough 



