244 



THE MAINTENANCE OF THE INDIVIDUAL 



strong supporting woody cells constitute the vascular bundles that 



put the root in connection with the stem and leaves above it. 



If mustard seeds, for example, are germinated in a moist chamber, 



a few days after germination the lower part of the root will be found 

 to be covered with a delicate, fuzzy growth. Ex- 

 amination of the root at this stage shows an actively 

 growing area of meristem, an elongating zone of tissue 

 directly back of it, with a zone of maturing tissue 

 above, which together make a zone of growth coincid- 

 ing more or less directly with an area covered with 

 fuzzy structures known as the root hairs. 



Root hairs vary in length according to their posi- 

 tion on the root, the longer ones being found at some 

 distance from the tip. They are outgrowths of the 

 outer layer of epidermis. A single root hair examined 

 under the compound microscope is found to be a 

 threadlike, almost colorless structure. The delicate 

 cellulose wall is lined by the 

 protoplasm of the cell, the 

 outer layer of which forms a 

 selectively permeable mem- 

 brane. Inside the root hair 



are found numerous vacuoles filled with cell 



sap. A nucleus is always present and may be 



found in the basal part of the cell, or in the 



hairlike portion itself. The root hairs are 



evidently living epidermal cells. 



An examination of a young plant growing 



in moist soil shows that the root hairs reach 



out between the particles of soil, apparently 



being closely cemented or attached in places 



to them. Each particle of soil is surrounded 



by a delicate film of water, which, with the 



dissolved minerals found in it, is absorbed 



into the root hair by the process of osmosis. 



The wall of the root hair is covered with a 



delicate layer of mucilagelike pecten formed 



by the outer layer of the cell wall and is also 



in contact with the moist protoplasm within 



the cell, which forms a delicate membrane 



Root hairs of 

 corn, showing 

 their relation to 

 the root tip. 



Root hair, showing its 

 relation to an epidermal 

 cell. How do you account 

 for the attachment of the 

 soil particles to the surface 

 of the root hair ? 



