40 Elementary Botany 



CHAPTER VI. 



DEVELOPMENT OF THE PLUMULE— FORMATION, 

 STRUCTURE, AND FUNCTIONS OF THE STEM. 



Whilst the radicle grows downwards to form the root of the 

 plant, the plumule is elevated above the soil, and produces the 

 stem, bearing leaves and other appendages. 



Popularly the stem is looked upon as differing from the root 

 in growing above the ground, but botanically there is a wider 

 difference. Underground portions of many plants, as the 

 Onion and Potato, which are generally called roots, are in 

 reality stems. 



The characters of stems as distinguished from roots are as 

 follows : — 



1. Their growing points are not covered with a root-cap, 

 but are surrounded by young leaves (buds). 



2. They have developed upon them appendages, variously 

 modified, but which differ in structure from the stems them- 

 selves ; whilst roots simply branch, the branches being repeti- 

 tions of the structure of the original root. 



3. Whilst, as we have seen, the branches of the root 

 have their origin in the deep-seated layers of the pericam- 

 bium (endogenous growth), the branches and appendages of 

 stems take their rise from more superficial layers (exogenous 

 growth). 



If we make a transverse section across the young stem 

 during the first year of its growth, we find that there is a great 

 difference in the appearance it presents in the two great groups 

 of Dicotyledons and Monocotyledons. 



Fig. 63 shows the section of a portion of a young dicoty- 

 ledonous stem. On the exterior there is an epidermis of 

 flattened cells. 



