266 ABSORPTION OF WATER 



entiated in the embryo. The primary root, which may be considered as a 

 downward extension of the main axis of the plant, gradually elongates, grows 

 in diameter, and produces lateral branches. Branches and sub-branches of the 

 primary root are called secondary roots. 



The primary root and its branches considered collectively are called the 

 primary root system. In the seed plants primary root systems develop only 

 from embryos. In many species the primary root system remains the only, 

 or at least the conspicuous root system throughout the life of the plant. In 

 perennial plants, especially certain tree species, such primary root systems may 

 attain an enormous size. 



All other roots, regardless of the organ of the plant on which they develop, 

 are termed adventitious roots. The roots which develop from bulbs, tubers, 

 corms, rhizomes, and cuttings are classed in this category. Adventitious roots 

 may even arise from the leaves of some species such as begonia, bryophyllum, 

 and walking fern. Such roots also develop from the lower nodes of the ver- 

 tical stems of many species, especially monocots. In some species, maize for 

 example, they may arise from nodes above the soil surface, becoming the so- 

 called prop roots. When they develop from stems adventitious roots most 

 commonly arise at the nodes. 



Two very generalized types of root systems which are often distinguished 

 are tap root systems and fibrous root systems. Practically all adventitious root 

 systems belong in the latter category. In the former the primary root system 

 is predominant, the primary root itself often being conspicuous. 



In many species, particularly in the monocot group, the primary root stops 

 growing and may even die while the plant is comparatively young. In such 

 species numerous adventitious roots originate in the region close to the base of 

 the stem resulting in the development of a root system of the fibrous type. 

 Other species of the monocot group, especially many grasses, develop numer- 

 ous slender adventitious roots from underground rootstocks. Such root sys- 

 tems are also distinctly of the fibrous type. 



The root sj'stem of a plant is often more extensive than its top. The 

 relative development of the top and roots of a plant is greatly influenced, 

 however, by a number of soil and climatic conditions (See discussion of shoot- 

 root ratio in Chap. XXXIV). 



The depth to which roots penetrate into soils is in part a species character- 

 istic, some species being typically more deep-rooted than others. However, 

 prevailing soil conditions usually exert a pronounced effect upon the depth of 

 penetration of roots. Rock strata are frequently so close to the soil surface 

 that the penetration of roots to any great depth is prevented. Similarly the 

 presence of hardpan or otherwise extremely tight layers of soil not far below 



