MODIFICATION OF ROOT SYSTEM 37 



Triglochin palustre, growing in similar situations, has sparse 

 adventitious roots coming in a bunch from the rather bulbous 

 base of the stem, without branches and only 2 or 3 in. long. 

 Ranunculus aquatilis has fine, short and sparingly branched 

 adventitious roots, Elodea canadensis has few, slender 

 adventitious roots which are quite unbranched and usually 

 not more than 4 in. long. Lemna produces a single 

 unbranched root about an inch long. 



Modification of the Root System. — From these descrip- 

 tions it will be seen {a) that the type and extent of the root 

 system are related to the conditions in which the plant lives, 

 although, even in peculiar and extreme environments, very 

 different types of root systems are to be found together ; 

 and {b) that the type of root system is to a high degree 

 specific. We must now consider the modifications which 

 the system of a species may undergo in response to environ- 

 mental conditions. The factors which principally affect 

 root development are : (i) soil texture, (2) soil moisture, 

 (3) the supply of salts, (4) hydrogen ion concentration, 

 (5) soil aeration, (6) soil temperature. In addition to these, 

 it must be remembered that the conditions to which the shoot 

 system is subjected, and the degree and rate of its growth 

 and development, must affect the growth of the root system. 

 Such effects can at present scarcely be disentangled from 

 the effects of the immediate environment of the root. 

 Further, it is often difficult or impossible to distinguish 

 in nature between the direct effects of soil moisture, aeration, 

 and texture on the root. 



I . The influence of soil texture is most obvious when a 

 layer of rock or of hard pan underlies the soil and prevents 

 penetration. Not only is the depth of the system thereby 

 limited, but changes in form may be caused, especially in 

 systems with tap roots, by the breaking up of the tap into 

 irregular branches. The presence of large stones and rocks 

 is a potent means of distorting the symmetry of root systems. 

 The great depth of the systems of prairie plants already 

 noted is, of course, only possible in an easily penetrable 

 deep soil. The stiffer clays may present considerable 



