108 GENERAL BOTANY 



cylinder have had time to develop. In other words, the herba- 

 ceous stem remains permanently in the twig stage of its woody 

 progenitors. 



With this brief discussion of the intermediate types of stem 

 structure, we may return to a summary of the differences between 

 strictly herbaceous stems like that of Salvia and the pure woody 

 types represented in our common trees. 



SUMMARY 



1. The corky bark of trees is absent in Salvia, and the epidermis 

 is continuous throughout the seasonal life of the plant. 



2. The cortex is wide and the pith is correspondingly large in the 

 herbaceous type in order that they may serve as storage areas. In 

 woody steins storage is more largely in the wood rays and paren- 

 chyma of the vascular cylinder, since the pith and cortex die early 

 in the life of such perennial plants. 



3. The vascular cylinder is thin in herbaceous plants, particularly 

 in the upper and younger parts of the stem. 



4. A cambium layer is present in most herbaceous stems, which 

 (as in Salvia) produces a complete vascular cylinder of xylem and 

 phloem in the older portions of the stem. 



5. Annual rings of growth are absent in the aerial stems of these 

 plants, since their life is limited to a single season. 



6. Wood rays are present in the xylem and phloem produced by 

 the cambium, but in many herbaceous stems they differ in structure 

 from those found in woody plants. 



MONOCOTYLEDONS 



The monocotyledonous stems, including corn, the cereals, the 

 grasses, and the palms, have a very different type of stem from 

 that of the dicotyledons described above. 



The distinctive features of the mature stem of monocotyledons 

 may be summarized as follows : 



The bundles (fibro vascular bundles) mentioned in the herbaceous 

 dicotyledon as forming parts of a dissected phloem and xylem ring 

 are scattered throughout the stem in typical adult stems of mono- 

 cotyledons (Fig. 57, A). The bundles themselves have also lost the 



