82 



Shedding of 

 Leaves. 



Develop- 

 ment of 

 Secondary 

 Members. 



Develop- 

 ment of 

 the Root. 

 Root- cap. 



which fit close together without intercellular spaces, gives rise 

 externally to a number of loosely united cells which often 

 protrude considerably beyond the surface. These areas of 

 loose cork cells are the lenticels, and they allow the necessary 

 interchange of gases to take place between the intercellular 

 spaces in the interior of the plant and the outer air. 



78. When leaves are about to 



be shed a layer of cork, continuous with the periderm of 

 the stem, is formed across the base of the petiole, which is, 

 however, penetrated by the vascular strands which descend 

 from the, leaf. Shortly after its formation a layer of cells, 

 a little distance above it in the petiole, becomes absorbed, 

 resulting in the entire separation of the tissues of the leaf 

 from those of the stem and the leaf accordingly falls off. The 



<_> / 



cork layer below the surface of the scar now protects the 

 inner tissues of the stem from injury and this protection is 

 completed by the blocking up of the cavities of the vessels 

 and sieve-tubes with gums or cork. 



79. Young leaves and branches 



first appear on the stem as minute humps caused by the 

 growth and division of a group of cells situated close below the 

 epidermis. As these cells grow the epidermis increases in area 

 accordingly but not in thickness, and it thus remains as a con- 

 tinuous layer over the leaves and young branches and stems. 

 Young roots, on the other hand, originate deep in the funda- 

 mental tissue of the parent root, just outside the vascular 

 bundles, and as they develop they force their way through 

 and rupture the tissue lying between them and the exterior 

 of the root, so that they emerge through slits in the tissue of 

 the parent. Each young root also, as a rule, originates just 

 outside a xylem strand of the parent, and they are therefore 

 found in straight longitudinal rows, there being usually as 

 many rows as there are xylem strands in the parent root. 

 These lateral roots also always first appear at some distance 

 behind the growing apex of the parent root, where the 

 tissues have already become differentiated 



80. The growing point of a root, 



unlike that of a stem, is protected by a mass of tissue 

 called the root-cap formed by the growth and division of 

 the external layer of cells at the apex. The outer cells of 

 this cap are continually exfoliated while new cells are con- 

 tinually added at the base. The actually elongating portion 

 of a growing root comprises a relatively small area, situated 

 immediately behind the apex, and as this elongates it pushes 

 the root apex, protected by its slippery conical cap, in front of 

 it into the earth like a shield. If the roots of a young 



