112 



BIOLOGY 



ference of the material from the outer part of the stem toward 

 the center, or the reverse. This type of stem is called an 

 exogenous stem (Gr. exo = outside + genes = a producing) , a 

 name given to it from the fact that it grows by the addition 

 of new layers of wood upon its outer side. Such a stem may 

 increase enormously in thickness; some trees live for many 

 hundreds of years and become several feet in thickness. 



There is, however, another type of stem which has a different 

 arrangement of the fibrovascular bundle. This is shown in 



cross section in Figure 52, 

 which represents a corn- 

 stalk. In this section there 

 is no ring of wood, the fibro- 

 vascular bundles are scat- 

 tered irregularly through 

 the stem, and there is no 

 bark or true pith. More- 

 over, closer examination of 

 these fibrovascular bundles 

 shows that they do not 

 have any distinct layer of 

 cambium cells. As a result, 



they have no growing layer and are not capable of increasing in 

 size. Such a stem is known as an endogenous stem (Gr. endon 

 = within + genes), and belongs to a type of plants, like the 

 grasses and bamboos, that grow tall and slender. Their stems 

 are only a little larger at the bottom than at the top and do not 

 materially increase in diameter. This type of stem forms a totally 

 different group of plants from the first, differing in many respects 

 in their leaves and flowers, as well as in their stem structure. 



STRUCTURE OF THE ROOT 



The structure of the root of the castor bean resembles that 

 of the stem, with some noticeable differences. A cross section 

 shows that the cortex is very much thicker than it is in the 



FIG. 52. CROSS SECTION OF 

 ENDOGENOUS STEM 



ep, the epidermis; /, the fundamental cells; 

 fb, the fibrovascular bundles scattered indefi- 

 nitely through the stem. 



