THE ANGIOSPERMAE : ROOTS 787 



in the woodland humus and can apparently live perennially in this fashion. 

 It only occasionally produces, endogenously, short, temporary flowering 

 stems, which serve for reproduction. 



Contrasted with these " plantless roots " are cases of " rootless plants," 

 especially among floating aquatics. Some of these which have dispensed 

 with roots are otherwise well developed, e.g., Utriculoria and Cerotophyllum, 

 but they also include Wolffia arrhiza, the most reduced of all the Angiosperms, 

 which consists of no more than a minute nodule of green cells. 



Roots are generally divided into two categories : (i) primary, normal 

 or soil roots, which originate from the embryo and usually persist throughout 

 life, and (2) adventitious roots, which arise secondarily from stem, leaf or 

 other tissues and which may be either permanent or temporary. 



The functions of the primary roots are to anchor the plant in the soil, 

 to absorb water and soluble substances and to serve as storehouses of food 

 materials. The functions of adventitious roots are very various. They may 

 sometimes enter the soil and assume the functions of primary roots. They 

 may even, as in the Monocotyledons, replace the primary roots altogether. 

 On the other hand, they may be modified into climbing organs, stilts or 

 props, thorns, boring organs in parasites and in other ways, which we shall 

 describe later. 



Root Systems. 



The whole extent of the roots of a plant is called the root system. The 

 development of this system differs fundamentally in Dicotyledons and in 

 Monocotyledons. 



Among Dicotyledons we find that the primary root of the seedling plant 

 develops directly into the main root, which is positively geotropic, i.e., 

 responsive to the influence of gravity by growing directly downwards. 

 The length and strength of its development depend on the life conditions 

 of the species of plant, and are very varied. In some cases its growth persists 

 throughout life and a large tap root is formed, equalling or exceeding the 

 main stem in size, and sometimes swollen into a storehouse of reserve materials. 

 In other cases its growth is slight and its duration is short, so that it is soon 

 lost sight of among the branch roots. The latter type is called the fibrous 

 root system. There is roughly an inverse ratio between the extent to which 

 the tap root develops and the amount of branching it displays. Normal soil 

 roots are scarcely ever entirely without branches, but in some storage tap 

 roots the branches are few and relatively small. 



Root branches are almost always endogenous, that is, they originate 

 from inner tissues (see p. 805) and bore their way through the outer tissues 

 to the surface. This imposes a delay on their appearance externally, so that 

 a distance of several centimetres may intervene between the apex of the 

 main root and the point of appearance of the first branch. This late appear- 

 ance of branches must have a considerable physiological advantage in the 

 case of a boring organ like a root and should probably be interpreted from 



