i8 



LECTURE II. 



become developed in moist air, they appear to be covered with a bnlhant white 

 velvety pile, which consists of the densely crowded root-hairs. These tubes are 

 simple protuberances of the outermost cortical cells of the root; they anse on the 

 recently developed part, and therefore behind the elongating region In a root 

 about I2-20 centimetres long, they cover, however, by no means the whole surface 

 but only a piece of a few centimetres in length; the root-hairs m fact die oflF 

 again after a few days, and completely disappear. It is therefore always only a 

 young, but completely elongated portion of the root, which is covered with vigorous 

 hairs. If we thus picture to ourselves a root as it grows longer, the posterior 



Fig. 10.— a seedling aiViaa Faba^ fastened by means 

 of a needle («) into a cork (not figured) to show how the 

 apex of the root (c) grows beyond the point of the needle. 

 The marks 5 and 10 are to be observed. Comparing B 

 with A, point 10 is not displaced at all, and 3 but little. 

 The growth has taken place chiefly between o and 5. 



Fig. II. — Seedling of Si7tapis Alba 

 (white mustard). A with the particles of 

 soil clinging to the root-hairs ; B after 

 their removal by washing in water. 



hairs die off, in proportion as new ones arise behind the growing apex; 

 thus the part of the root provided with hairs moves forward. If this takes 

 place in the earth, it is obvious that the portion beset with vigorous root-hairs 

 continually comes in contact with such particles of soil as have before' remained 

 untouched. Now, it is the root-hairs by which the roots become capable of actually 

 taking up the water and the nutritive matters of the soil. As these fine tubes grow in 

 between the particles of soil, they apply themselves here and there to them so 

 intimately and firmly, that they cannot be separated again without injury. How 

 necessary this is, I shall be able to make quite clear in the theory of nutrition! 

 so much, however, is understood without further remark, that by means of this 

 arrangement, particles of earth which we may imagine to ourselves in the form of 



