1876.] 



AND HORTICULTURIST. 



213 



ROOT HAIRS. 



BY BYRON C. HALSTED, BUSSEY IKSTITUTE, MASS. 



That the largest portion of the Uquid used by 

 the growing plant makes its entrance through 

 the roots, from the soil, is a well-established fact; 

 but those parts which are the most active in the 

 absorption of this food material in solution were 

 for a long time not so clearly understood. 



By careful experiments and microscopic in- 

 vestigation, it is found that the extreme tips of 

 young roots are about the only portion which 

 take little or no part in this work. A short dis- 

 tance ba^k from the growing points, on nearly 

 all grooving roots, may be seen with the aid of a 

 microscope a large number of minute, slender 

 bodies extending out in all directions from the 

 surface of the root. These thread-like structures 

 are not unaptly called root hairs, and consist of 

 sac-like protuberances,as outgrowths from the epi- 

 dermis or surface cells of the root. With the 

 naked eye they are not easily seen, but their 

 presence may be inferred from the manner in 

 which they cling to the jjarticles of the soil when 

 a young root is lifted carefully from the earth in 

 which it was growing. This power which they 

 have of fixing themselves to the grains of earth 

 is very great; so that when a plant is taken vio- 

 lently from the soil, a large part of these deli- 

 cate hairs are broken from the roots and retain 

 their attachment to the soil. As the root grows 

 along in the earth new hairs are produced while 

 those behind perish as the root becomes woody, 

 and a dense, non-absorbing, protecting epidermis 

 is formed; so that the active life of a single 

 hair is of short duration. The office of these 

 hairs must have already suggested itself to the 

 reader. By means of these prolongations the 

 greater part of the absorption takes place, though 

 the newly formed surface cells are also active. 

 But the surface which they expose is small in 

 comparison with that of the hairs. It is hard to 

 conceive of a more thorough and economical 

 means of exposing an absorbing surface, at the 

 same time keeping in view their method of ap- 

 ical growth, required strength, and a means of 

 rapid transfer of its liquid through a tube to the 

 root to which it is attached. 



In a poor soil roots run rapidly in all directions, 

 and are often very long ; so, with the hairs, they are 

 put out quickly, take in what nourishment they 

 find and soon die. While on the other hand, in 

 a rich soil the roots are not required to be long, 

 and the hairs are of greater duration. Here is 



tilizers which, because below ground and out of 

 sight, is apt to be overlooked. 



A consideration of the root hairs involves a 

 question about which there has been much the- 

 orizing and speculation, viz : — how do these root 

 and root hairs take in the liquid from the soil ? 



Putting aside these various notion!*, perhaps it 

 will not be amiss to state very briefly the view 

 now generally held, though still thought by some 

 investigators not to explain every point. It is 

 a well-known fact that solid, porous bodies have 

 the power of taking up liquids to a greater or 

 less extent, according to their nature and sur- 

 rounding circumstances. A dry cloth hung so 

 that one corner will dip into water will in a .short 

 time become saturated. This is sometimes called 

 capillary attraction, and has a part in the root 

 absorption. From an extended study and knowl- 

 edge of the properties of liquids the law of diffu- 

 sion has been established, viz : — when two or 

 more misible liquids of different degrees of den- 

 sity are placed in contact, interchange will take 

 place until, when the diffusion is complete, the 

 whole liquid will be homogeneous. Instances of 

 partial diffusion are too familiar to warrant 

 space for illustrations. This property of liquids 

 will account for the movement of the absorbed 

 sap to any part of the same cell^from the tip 

 of the hair to the basal portion. But there is 

 another kind of diffusion, first nicely pointed 

 out by Dutrochet and afterwards largely experi- 

 mented upon by Graham, which has received 

 the name of osmose, or membrane diffusion. 

 When liquids differing in density are separated 

 by a thin membrane, as a bladder, diffusion takes 

 place through this septum with a rapidity de- 

 pending on the nature of the liquids and sep- 

 parating membrane, the greater flow being to- 

 wards the denser fluid. This intercepting mem- 

 brane often greatly accelerates the diffusion, but 

 just how it acts, and the chemical and other 

 changes which take place in it, or on its surface, 

 are not well known; still the fact is very striking 

 when, with the proper adjustment of the appa- 

 ratus and materials, liquids can be made to rise 

 through a tube to the height of many feet. The 

 cell wall of a root hair is such a membrane, sep- 

 arating the denser liquid within the cell from the 

 weaker one without, and this membrane is a 

 living, growing one, and may for that reason be 

 much more effective for osmotic action. 



It is difficult to state what portion of the work 

 is done by each of these forces, but combined, 



a saving fi-om the use of manure and other fer- ' they seem to answer the questions concernmg 



