CHAPTER XXIX 

 ROOTS: DEVELOPMENT AND STRUCTURES 



To complete the story of the water and salt relations of plants we shall 

 have to examine the underground parts of plants that have roots. The 

 soil is a part of the immediate environment of these plants. The water in 

 the upper layers of the soil, where most roots grow, is transitory. It enters 

 from rain and melting snow and moves out by seepage, by evaporation, 

 and through the plant. Only a portion of it is held in the interstices 

 among the innumerable particles of the soil. From these interstices the 

 water moves into the roots and upward into the stems and leaves and 

 then into the air by transpiration. We have seen that in many plants the 

 amount of water that passes through them during a growing season is 

 large in comparison with either the volume of the plant or the water 

 necessary for photosynthesis and other plant processes. How such large 

 amounts of water can enter the plant from the soil will be better compre- 

 hended when we see how roots develop and become distributed among 

 the soil particles. 



Types of roots. When the seeds of most plants germinate, the first part 

 of the embryo to enlarge and push through the seed coats is the hypo- 

 cotyl, having at its lower end a root primordiiim. The hypocotyl is the 

 part of the embryo between the cotyledons and the primary root, and is 

 really the base, or the first-formed part of the stem of many plants 

 (Fig. 8). This initial expansion of the embryo is due to cell enlarge- 

 ment. The hypocotyl may ultimately elongate from a small fraction of 

 an inch to several inches shortly after germination. Following the emer- 

 gence of the hypocotyl from the seed coats, the cells at the lower end 

 of the root primordium begin to divide, and from this primordium the 

 primary root develops. The hypocotyl appears to be lacking in the 

 embryos of some plants, such as the grasses (Fig. 113). The plumule 

 appears to be separated from the primary root principally by a node. If 

 a hypocotyl is present in this type of embryo it is quite small and does 

 not elongate when the embryo germinates.^ 



^ The term radicle is variously used by different writers to refer either to the rudimentary 

 root of the embryo or to both the root and the hypocotyl. 



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