THE ANGIOSPERMAE : ROOTS 789 



diarch roots, which normally have four rows. These rows can be seen if a 

 well-branched root is examined endways. The arrangement is due to the 

 place of origin of the branches, as we shall see later. 



Root production is not equally vigorous all the year round. In bulbous 

 and other plants with pronounced seasonal activity, root production usually 

 marks the beginning of the active period. Among trees there are generally 

 two maxima, one in spring before the opening of the leaves and one in 

 autumn, from September till the first frosts. Midsummer and midwinter 

 are resting periods. 



The amount of branching is greatly influenced by the aeration of the 

 rooting medium. Roots in water or in mud are usually sparingly branched 

 and sometimes not at all. Even in ordinary soils a great difference exists 

 between wet and dry layers, and a deeply rooted plant, which may pass 

 through several different soil zones, may have several distinct layers or storeys 

 of maximum root development, corresponding to the drier and better aerated 

 zones in the soil. 



Two distinct types of root system have been observed among woody 

 perennials : the extensive and the intensive. In the former the main root 

 branches grow rapidly with little thickening, and the smaller branches are 

 short lived, so that young roots are only to be found at the periphery of the 

 system. In the intensive type branching is freer and the branches themselves 

 are shorter and more persistent, so that the system contains young roots at 

 all parts and not only at the periphery. 



There is no fixed proportionality between the extent of root and of shoot 

 development. It is greatly influenced by external conditions. The roots of 

 dicotyledonous trees have a spread which is roughly equal to the spread of 

 their canopy. Most annuals have, on the other hand, a very restricted root 

 development. Xerophytes show an extreme disproportion, their root systems 

 being often thirty to forty times as extensive as their shoot systems. 



It is a matter of extreme difficulty to extract a root system from the soil 

 in anything like its entirety, so that our information on the total extent of the 

 system is limited to a few cases. The work of Weaver in America has, how- 

 ever, shown that it is much greater than used to be supposed. By carefully 

 washing away the soil from the sides of deep trenches, he was able to map 

 complete sections through the root systems of a number of species. Examples 

 of these are shown in Fig. 785. Such sections show that penetration to a 

 depth of 6 or 7 ft. is by no means uncommon and that even a small plant may 

 permeate several cubic yards of soil with thousands of fine rootlets. Lateral 

 movement of water in the soil is normally very restricted, and the plant's 

 chances of gaining a sufficiency of water in dry weather and of food at all 

 times depend upon the completeness with which it can occupy every available 

 portion of the soil volume. Dittmer has calculated that a single Rye plant 

 under favourable conditions may develop a root system with a total length 

 of 350 miles and with a total surface, including hairs, of 7,000 sq. ft. Pavly- 

 chenko has estimated that the length of the root system produced by an 

 isolated Wheat plant growing without competition is 45 miles. When 



