38 THE BIOLOGY OF FLOWERING PLANTS 



resistance to penetration. But in the case of clays the exact 

 cause of the effects is at once obscured by the different 

 conditions of moisture and aeration. 



2. Soil Moisture. — The necessities of the plant would, 

 it might seem, require the development of a root system 

 proportionately more extended the drier the soil. On the 

 other hand, increasing drought means a decrease in the 

 materials and conditions favourable to vigorous growth. 

 As a result it seems that plants growing in extremely dry 

 and extremely wet soil both produce relatively poor root 

 systems, though in the latter case the total growth of the 

 plant is much better. In intermediate moistures more 

 extensive root systems occur. From his own observations, 

 Freidenfeldt (1902) draws the conclusions : (i) that a plant 

 shows a maximum root development at a particular water 

 content for a given type of soil ; (2) that on each side of this 

 root development is diminished ; (3) that dry soils tend to 

 promote growth in length of the tap or chief laterals and so 

 to produce a deep root system ; (4) but that many native 

 plants which grow both on very wet and very dry soils, such 

 as Nardus stricta, Festuca rubra, species of Saxifraga, and 

 of Alchemilla, show relatively small differences in the two 

 kinds of habitat. These results, however, must be taken 

 with a certain amount of caution, for other conditions than 

 soil moisture are certainly operative. 



Weaver (19 19) examined the root systems of examples 

 from two or more different stations of each of eleven species ; 

 in seven there were striking modifications of root habit. 

 Various factors are concerned, but Weaver (19 19, 1920) lays 

 chief stress on the differences of water supply. In the plains. 

 Euphorbia mouiaua (Fig. 4) had a root system penetrating 

 to 7*5 ft. with a working depth of 2 to 4 ft., while in the half- 

 gravel slide the maximum depth was 4*5 ft., and the whole 

 of this layer of soil was filled with rootlets. In general 

 the most marked development of fine roots occurs where 

 moisture supply is most favourable. 



Certain species, however, such as Koeleria cristata with 

 shallow branch roots and Allionia linearis with a deep 



