CHAPTER II 

 THE ECOLOGY OF ROOTS 



Roots are primarily the absorbing and anchoring organs of 

 plants. Internally their structure is similar to that of stems. 

 Externally they are distinguished from stems by the absence of 

 nodes and internodes and especially by the absence of regularly 

 arranged buds. They vary greatly in form and size. Some of the 

 variations are due to inheritance while others are due to the environ- 

 ment and so are of ecological significance. 



Because the root system of the plant is, for the most part, hidden 

 within the soil, it has not received as much attention from students 

 as the more obvious aerial portion. But the aerial portion of a 

 plant cannot continue to live and grow without the root system, 

 any more than the roots can live and grow without the stem sys- 

 tem, and it is necessary to understand the ecological relations of 

 all parts of the plant before we can successfully apply our knowledge 

 to practical problems involved in the growing and utilization of 

 plants. 



5. Absorption.— The absorption of materials from soil takes place 

 primarily through root hairs. Root hairs are extensions of cortical 

 cells of roots. They are ordinarily from one to several millimeters 

 in length and so small that as many as 300 have been counted on a 

 single square millimeter of surface, though often they are not nearly 

 so numerous as that. The numerous root hairs increase the absorb- 

 ing surface of the roots from 2 to 10 times and sometimes even 

 more and this is probably the chief advantage of these structures. 

 Most root hairs haA'e cell walls which are ^ery thin and pliable and 

 possess a mucilaginous material in the outer part of the wall, and 

 this makes it possible for them to come into \ery close contact 

 with the soil particles. They usually become ^■ery irregular in 

 shape through this intimate contact with the soil particles and they 

 cling to these particles so tenaciously that when roots are pulled 

 from the soil the root hairs are for the most part broken off from the 

 root rather than being pulled loose from the soil particles. 



The ordinary thin-walled root hairs are rather ephemeral struc- 

 tures lasting only a few days or at most a few weeks, depending 

 upon the kind of plant and upon the environment in which the 

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