ABSORPTION NUTRIMENT. 2"] 



heat, and weigh again. It has lost in weight. This has been brought about 

 by driving off the moisture which still remained in the soil after the plant 

 began to wilt. This teaches that while plants can obtain water from soil 

 which is only moist or which is even rather dry, they are not able to withdraw 

 all the moisture from the soil. 



56. Acidity of root hairs. — If we take a seedling which has 

 been grown in a germinator, or in the folds of cloths or paper, 

 ■so that the roots are free from the soil, and touch the moist root 

 hairs to blue litmus paper, the paper becomes red in color where 

 the root hairs have come in contact. This is the reaction for 

 the presence of an acid substance, and indicates that the root 

 hairs excrete certain acids. This acid property of the root hairs 

 serves a very important function in the preparation of certain 

 of the elements of plant food in the soil. Certain of the 

 chemical compounds of potash, phosphoric acid, etc., become 

 deposited on the soil particles, and are not soluble in water. 

 The acid of the root hairs dissolves some of these compounds 

 where the particles of soil are in close contact with them, and 

 the solutions can then be taken up by the roots. 



57. This corrosive action of the roots can be shown by the well-known 

 experiment of growing =• plant on a marble plate which is covered by soil. 

 After a few weeks, if the soil be washed from the marble where the roots 

 have been in close contact, there will be an outline of this part of the root 

 system. Several different acid substances are excreted from the roots of plants 

 which have been found to redden blue litmus paper by contact. Experiments 

 by Czapek, however, show that it is carbonic acid which has the power of 

 dissolving these compounds, while the other acids excreted by the roots do 

 not have this power. 



