ROOT HAIRS AND GROWTH 



345 



cell wall, but there is normally no thicken- 

 ing of the wall, nor any apparent differen- 

 tiation of structure or substance within the 

 root hair during its elongation. 



Not only are root hairs desirable ob- 

 jects for use in the study of cell enlarge- 

 ment, but they also afford one of the most 

 accurate means of ascertaining the bio- 

 logical effect of various physical and 

 chemical agents. They can be readily 

 exposed to different types of light and other 

 radiations. Experiments can be arranged 

 so that temperature can be accurately con- 

 trolled and its effects investigated upon a 

 cellular basis. Root hairs can likewise be 

 subjected to various types of electrical 

 phenomena. 



It is, however, in the realm of chemical 

 agents that they lend themselves to the 

 greatest diversity of experimentation. The 

 chemical substances which may be pre- 

 sented to roots of plants may be classified 

 into four groups according to their effect 

 upon the plant. They are either nutritive, 

 that is, actually utilized in the construc- 

 tion of necessary organs and compounds; 

 or they are toxic, that is, they have an 

 injurious effect upon the rate of plant proc- 

 esses or cause abnormal development; or 

 they are stimulating, that is, they acceler- 

 ate plant processes above the normal; or 

 they are neutral, that is, have none of 

 these effects. 



Root hairs are especially well adapted 

 for the study of the biological effect of 

 chemical elements and compounds, in that 

 they grow either in air, so that gases may 

 be studied, or in solutions, so that in- 

 organic and organic soluble compounds 

 may be used. The only element that is 

 necessary in addition to those in water is 

 calcium. Therefore they can be grown in 

 so simple a solution as calcium hydroxide. 

 Other soluble substances can be studied 

 by adding them to calcium hydroxide, and 

 hence having them in a very simple solu- 



tion. In this way the various nutritive, 

 toxic, and stimulating agents can be 

 thoroughly studied as to their biological 

 effect in various concentrations and under 

 various conditions of light and tempera- 

 ture and of degrees of acidity and al- 

 kalinity of the medium. 



In studying these chemical factors it is 

 obviously advisable to investigate the 

 nutrient constituents first. Only after 

 they are completely understood can we 

 hope to make a careful analysis of the 

 stimulating and toxic substances. We 

 know that certain elements, namely, nitro- 

 gen, potassium, phosphorus, magnesium, 

 iron, sulphur, and calcium are actually 

 utilized in the construction of plant 

 tissues. These are certainly to be re- 

 garded as nutrient elements. Certain 

 others, such as sodium, chlorine, manga- 

 nese, and boron seem to be present in 

 most plants, and may eventually prove to 

 be of direct nutritive value. 



The study of the nutrient constituents of 

 plants began with the chemical analysis of 

 wild plants and of the soil upon which 

 they grow. As the science of chemistry 

 has developed this study has become more 

 and more intensive and extensive. The 

 study of cultivated crops and fields was 

 undertaken in the same way with more 

 dependable results because of the more 

 uniform conditions. If such investiga- 

 tions were made in experimental plots 

 especially planned for this purpose still 

 more consistent data were obtained. A 

 further improvement consisted in growing 

 the plants in greenhouses in benches, 

 flats, or pots, where temperature and 

 humidity could be somewhat controlled. 

 But even here the complexity and vari- 

 ability of the soil made definite conclu- 

 sions very difficult. A very important 

 advance was then made by growing the 

 plants in sand to which nutrient elements 

 were added in the form of simple inor- 



