12 



PEACTICAL BOTANY 



-P 



FIG. 9. Diagram of a cross section of 

 a geranium stem 



The regions are the outer bark (6)*, the 



cortex (c), the woody tissue (w), and the 



pith (p) 



are intermingled with the soft pith tissue. There is a large 

 group of plants, the monocotyledons, which have the irregular 

 distribution of bundles just described. In such stems there is 



usually, around the outside, a 

 much harder tissue, which is 

 extremely strong, and which 

 serves to strengthen the stem. 

 In other kinds of plants a 

 cylinder of bundles is defi- 

 nitely arranged about the pith 

 (Fig. 9), and this arrangement 

 is also characteristic of a great 

 group, the dicotyledons. Other 

 features of these groups are 

 discussed in later chapters, 

 and in this connection it is 

 important only to note some 

 general characters of the stem. 

 13. Stems and their work: annual growth. In many of our 

 common annual plants (those that live for but one year) the 

 arrangement of the bundles or woody tissue in the form of a 

 cylinder about the pith is readily seen. In such plants the pith 

 usually occupies more of the stem than does the wood. The 

 proportion of pith to wood is much less in the perennial plants 

 (those that live for two or more years). In a cross section of 

 one of the common trees, unless the specimen be quite young, 

 it will be difficult or impossible to discover any pith region. 

 The greater part of the section is made up of wood. Each 

 year there is formed a layer of this woody tissue from the 

 inner, heavy-walled cells of the bundles, which persist and 

 give strength and support to the whole tree. The great size 

 of our forest trees is made possible by this arrangement of 

 bundle tissues. The record of growth may be read by study- 

 ing the rings of wood. The amount of a year's growth and 

 the total time that a tree has lived can be reckoned. You will 

 also find it interesting to study the top of your desk or the 



