36 DISEASE IN PLANTS 



them to be delicate prolongations of the root- 

 surface to facilitate the absorption of water. 



The real importance of these organs, however, 

 has only become apparent since Sachs, in 1859, 

 recognised their relations to the particles of soil 

 between which they extend and to which they 

 cling. 



In 1883 Schwarz made a very thorough study 

 of their biological character, and in 1887 Molisch 

 gave us new facts as to their physiology. Our 

 knowledge of them has been rendered very much 

 more intimate by the researches of Pfeffer and 

 De Vries on osmotic and plasmolytic phenomena, 

 and they serve as an excellent study of some of 

 the best results of modern physiology. 



In the normal case, such as is exemplified 

 by a seedling wheat or bean, the root-hairs 

 arise some distance behind the growing tip of 

 the root, an obvious adaptation which prevents 

 their being rubbed off by the soil, as they 

 would be if developed on parts still actively 

 lengthening. As those behind die off, new ones 

 replace them in front, and so we find a wave of 

 succession of functionally active root-hairs some 

 little distance behind the tip of the root : the 

 same order of events holds for each new rootlet 

 as it emerges from the parent root, and so suc- 

 cessive borings in the soil, made by the diverging 

 root-tips, are thoroughly explored by these root- 

 hairs. 



Measurements have shown that in various 

 plants the surface of root on i mm. of length is 



