ABSORPTIOiV NUTKIMEA'T. 2/ 



heat, and weigh again. It has lost in weiglit. 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 raoist 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 salt, and indicates that the root hairs ex- 

 crete certain acid substances. This acid property of the root hairs 

 serves a verj' 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 a 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 show, however, that the carbonic acid excreted by the roots has 

 the power of directly bringing about these corrosion phenomena. The acid 

 salts are the substances which are most actively concerned in reddening the 

 blue litmus paper. They do not directly aid in the corrosion phenomena. In 

 the soil, however, where these compounds of potash, phosphoric acid, etc., 

 are which are not soluble in water, the acid salt (primary acid potassium phos- 

 phate) which is most actively concerned in reddening the blue litmus paper 

 may act indirectly on these mineral substances, making them available for plant 

 food. This salt soon unites with certain chlorides in the soil, making among 

 other things small quantities of hydrochloric acid. 



XoTE. — It should be understood that food substances in solution, during 

 absorption, diffuse through the protoplasmic membrane independently of each 

 •other and also independently of the rate of movement of the water from the 

 soil into the root hairs and cells of the root. 



