NATURE OF PLANTS 87 



grafted upon a branch of a peach tree will develop a normal shoot 

 bearing plums and the peach tree will also continue its growth 

 unaffected. Ordinarily only closely related forms may be grafted, 

 as tomatoes and potatoes, apples and quinces, etc. 



42. Comparison of Stem and Root. — It will be noticed that the 

 structure of the growing tip of the stem presents several strik- 

 ing departures from that of the root. In the first place a pro- 

 tective structure such as the root cap is not required (Fig. 61). 

 The leaves, and consequently the branches are formed with great 

 regularity from the superficial tissues of the stem, whereas the 

 rootlets are deep seated in origin and irregularly distributed. 

 Furthermore, the elongation of the stem is distributed over several 

 centimeters, while in the root it is confined to a few millimeters. 

 The leaves and branches are distributed over the entire zone of 

 elongation, while lateral roots do not appear until elongation has 

 ceased. This prolonged growth of the tip of stems is of great 

 advantage in bringing about the proper adjustment of leaves and 

 branches. These organs come into sharp competition with each 

 other as well as with other plants, and so long as growth continues 

 there is a possibility for them to become adjusted to the light 

 without interfering with one another. The mode of elongation 

 of stems and roots is essentially alike. Each has a formative 

 region at the apex characterized by active cell formation and 

 slight elongation, back of this is the zone of rapid elongation and 

 following this is the zone in which elongation is gradually ceas- 

 ing and the tissues are assuming their characteristic forms and 

 functions. It is also important to note that the growth of the 

 cells on one side of the stem or root is faster at a certain time 

 than any of the other cells. This will cause the stem to be bent 

 towards the opposite side. This condition of rapid elongation 

 does not continue on one side for any considerable time but 

 slowly moves from point to point around the stem so that the 

 stem is successively bent from side to side, thus causing the 

 apex to travel in an irregular circle. By these constant move- 

 ments, nutations, the stems and roots are more fully exposed to 

 the various forces that influence their development and conse- 



