78 STRUCTURAL BOTANY 



protected by the leaves of the terminal bud, which close 

 in over it. The growing-point of the root requires pro- 

 tection even more, for it has to make its way between 

 the hard particles of the soil, and hence a special organ 

 is present which performs this sheltering function. This 

 organ is the root-cap, a sheath of tissue shaped rather like 

 a thimble, which envelops the delicate growing apex. 

 As the root elongates, the cap. is pushed before it, and 

 has to bear the brunt of the friction with the soil (see 

 Figs. 33 and 34, c). The outer layers of the root-cap 

 become gradually converted into mucilage, which serves 

 both to diminish the friction and to keep the apex moist, 

 and as this happens, the cap is renewed from within by 

 the growing point itself, which thus has to provide 

 new tissue for the cap in front of it as well as for 

 the body of the root behind (see Fig. 33, c). 



The presence of the cap is another very constant 

 character by which roots are distinguished from stems. 



d. Transition from Stem to Root 



It has already been said that the tissues of the 

 root are continuous with those of the stem. We will 

 now see how the one organ passes into the other. In 

 order to understand this, it will be necessary to study 

 a fairly young seedling, before there has been much 

 growth in thickness. 



The first two leaves which the Wallflower seedling 

 forms are placed opposite to one another, as is almost 

 always the case in the class of which it is a member. 

 These two first leaves are called the seed-leaves or 

 cotyledons, and differ considerably from the other leaves 

 of the plant. For example, they are nearly round in 



