*^ "^^^ NATURE OF PLANTS 49 



offered them the root hairs soon perish and with them the plant. 

 Continued absorption is only possible so long as the root continues 

 to grow. For this reason a plant showing signs of starvation is 

 removed from the jar, the tangled mass of roots next to the jar 

 is cut off together with a portion of the earth and then repotted 

 with fresh earth. In this way room is given for the rootlets to 

 grow out into the earth and develop a new absorbing surface and 

 additional materials are placed at the disposal of the root. We 

 see now that transplanting of the plant should be done in Septem- 

 ber or October or in the early spring in order to permit the rootlets 

 which renew their growth very early in the spring to establish in 

 the soil new absorbing surfaces before the leaves develop and bring 

 about an excessive loss of water through transpiration. Why are 

 plants often pruned or cut back in transplanting? 



You have often noticed that fields of grain and other crops turn 

 yellow and die during a long rainy season. This is due to the 

 fact that the air spaces between the earth particles become filled 

 with water and there is no longer possible an interchange of gases. 

 At least two-fifths of the space in these capillary pores should be 

 filled with air to insure healthy plants. This is particulary notice- 

 able in clay soils which are referred to as cold, wet, and sour soils. 

 The reason of this is that the earth particles of such soils are very 

 fine and consequently compact and the small capillary spaces be- 

 tween the mineral particles pull up and hold the water, with great 

 tenacity. Consequently acids from decaying vegetation and CO2 

 from the roots as well as water accumulate in excess and render 

 the soil unwholesome. In this connection note should be made 

 of the suggestive work that is being accomplished today tending 

 to show that roots give off substances, probably of organic nature, 

 w^hich are toxic in character and so render the soils unfit for plant 

 growth . These substances are readily oxidized and rendered harm- 

 less. The presence of oxygen and certain fertilizers increase the 

 oxidising power of the roots. While this theory is far from being 

 established it is possible that we may have a new explanation for 

 the sterile condition of some soils ; also plowing and the use of fer- 

 tilizers may have a new" meaning. Clay soils, while richer in 

 plant foods than other soils require an admixture of sand and an 

 5 



