[Chap. XVIII RESPIRATION AND PLANT DEVELOPMENT 161 



probably act mainly as catalysts. Of the external factors, temperature, 

 oxygen, and sometimes water occupy a place of first importance under 

 natural conditions, and also in the cultivation and care of plants/ Under 

 experimental conditions many factors are known either to increase or 

 to decrease respiration in plants. 



The relation of respiration to plant development and to certain prac- 

 tices in the care of plants and plant products. Plants often fail to survive, 

 or tlieir growth is retarded, merelv because of a lack of sufficient oxvgen. 

 The aerial parts above ground are exposed to an abundance of oxygen 

 in the surrounding atmosphere, but some parts are exposed onlv to the 

 oxygen that is in the air of the soil. The parts of land plants that are 

 underground (roots, underground stems, and also seeds and seedlings) 

 may be deprived of sufficient oxvgen when thev are flooded bv water, or 

 when the ground water level rises too near the surface of the soil. The 

 water per se is not harmful, but it excludes nearly all the free oxygen, 

 and death by suflFocation is the result. Less oxygen is likelv to be avail- 

 able to the plant when the water is stagnant than when it is flowing. 



Perhaps everyone is familiar with the fact that manv common plants 

 grow only where their roots are in well-drained soils, while others may 

 grow where the soil is saturated or even submerged. This difference 

 may be due to the fact that some plants, like some animals, can sur\'i\e 

 with less free oxygen than others. The roots of the black willow, for 

 instance, will grow in soils almost wholly deprived of free oxvgen, 

 whereas the roots of sugar maple, scarlet oak, and man\' other trees 

 will die of suffocation. However, the roots of most land plants grow well 

 in water that is continuouslv aerated. 



Some interesting ideas of the growth and distribution of plants in rela- 

 tion to free oxygen may be gained bv observing the zones of plants on 

 the wet shores of shallow lakes and in low places in fields and woods 

 (Figs. 267, 268). Pondweeds, cattails, and bulrushes grow with their 

 roots continuously submerged in water, and some have roots in muck 

 where there is almost no free oxygen. In some of these plants large in- 

 ternal air passages extend from the leaves and stems all the way to the 

 root tips. The submerged roots of such plants may not survive unless 

 the oxygen that diffuses downward from the air-exposed leaves is ade- 

 quate. The underwater parts of these plants drown, or suffocate, if the 

 tops are cut off below the water le\'el. 



^ Light should also be recalled in this connection because of its indirect effects on 

 respiration through the amount of food made, the opening of stomates, and the raising of 

 the internal temperatures. 



