CHAPTER II 



moisture from them appears on the sides of the jar in a few minutes. 

 A piece of polished marble in the bottom of a box of growing seed- 

 lings will become etched by the acid secretion from the root-hairs. 

 Probably these simple experiments with the deductions to be drawn 

 from them will occupy as much time as the plant lessons are entitled 

 to in primary schools. The teacher who can find time for more will 

 find them suggested in the more recent books mentioned in the pre- 

 face, particularly in Bergen and Oels. 



As soon as seeds absorb moisture the insoluble stored food is 

 changed to soluble forms, and other chemical changes that always 

 accompany growth and nutrition occur. As stated in the supplement 

 to the first chapter, these chemical changes require oxidation, that is, 

 the plant respires. Higher plants have not, like higher animals, 

 special organs for respiration, but the process is going on over the 

 whole of the plant throughout its existence. Whether the respira- 

 tion of plants should be taught in the grammar grades seems to me 

 very questionable. The experiments bearing on this subject require 

 some knowledge of chemistry and are not likely to be comprehended. 

 Respiration, too, is to a considerable extent the reverse of photo- 

 synthesis, and if there is an attempt to teach both processes the whole 

 subject becomes confusing. 



Of the experiments suggested above, by far the most impressive one 

 is growing corn in moist air. The root-tips and root-hairs are beauti- 

 fully distinct. The teacher has only to remember that the root-hairs 

 are so delicate that they shrink after a few minutes of exposure to dry 

 air. The relation of the number of hairs to the plant's facilities for 

 obtaining moisture is very interesting. The mustard seeds germinat- 

 ing on gauze, and the wheat on the sponge, produce very striking 

 root-hairs so long as they are in contact with moist air only, but when 

 the roots have reached the water in the glass they cease entirely to 

 develop hairs; that is, root-hairs are abundant so long as moisture is 

 difficult to obtain, but they are obviously unnecessary to roots 

 immersed in water. By actual count there are from ten to four 

 hundred root-hairs per square millimeter. Root-hairs on plants grown 

 in the soil become very firmly united with it, so that it is impossible 

 to entirely shake off, or even wash off, adherent particles. This adhe- 

 sion is due partially to the solvent acid of the hairs, but it is also 

 because of the avidity of their cell contents for the molecules of water 

 adhering to molecules of the soil. The absorbent cells must be able 

 to extract considerable water from soil that appears dry. Root-hairs 

 perish on the older parts of roots, but there is always a zone of 

 absorbent cells just back of the tip. 



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