46 THE BIOLOGY OF FLOWERING PLANTS 



Of alteration in the form or depth of root systems 

 caused by temperature nothing is known. Mention may 

 be made here of the view expressed by Diels (1918a), that 

 the occurrence of a thick layer of light-coloured cork about 

 the root neck and stem base in the desert plants of West 

 Australia tends to protect the living tissues from too great 

 overheating in the hot sand. 



Interacting Conditions in Nature. — The system which 

 actually develops in any given case in nature is the resultant 

 of the interplay of these various factors, acting directly or 

 indirectly on the growing regions of the roots, influenced of 

 course by the development of the shoot, and working always 

 on the material of the plant's inheritance. We have quoted 

 instances where the root system was very plastic, external 

 conditions changing it readily and profoundly, and others 

 where it remained very constant under widely different 

 circumstances. Various instances of modifications under 

 natural conditions have been given, in the main such as 

 seem to indicate the working of one particular factor. We 

 may close this account with one or two cases where, as is 

 usual, the active factor cannot be easily identified, or where 

 more than one is involved. 



Yapp (1908) describes two plants of Lysimachia vulgaris 

 growing near each other in Wicken Fen". Numerous 

 strong adventitious roots arise from the horizontal rhizomes. 

 In the first plant the rhizome lay i| in., in the second 

 3I in. deep. In the first the roots grew vertically down, in 

 the second they spread out obliquely or almost horizontally. 

 It is probable that we have here an effect of diminished 

 aeration due to approach to the water-level. 



In deposits of river sand thin layers of hummus soil may 

 often be found interbedded. The roots of trees and shrubs 

 tend to be confined to such layers. In an exposed section 

 the way in which the roots, e.g. of a birch, run along the 

 dark-coloured humus is sometimes striking. Waterman 

 (19 1 9), in a study of the dune plants of Lake Michigan, 

 found that the only cause of irregularity or asymmetry in 

 the root systems was the presence of nests of humus, in 



