PLANT ORGANS AND ORGANISMS 139 



There are two plans of branching in trees. When the trunk, or 

 main stem, extends vertically upward to the tip, as it does in the 

 junipers, spruces and other conical trees, the type of branching is 

 called excurrent] when it divides into several more or less equal 

 divisions as in the elm and other spreading trees, it is said to be 

 deliquescent. The deliquescent plan is the more common one among 

 our deciduous trees. 



An herb is a plant whose stem does not become woody and perma- 

 nent, but dies, at least down to the ground, after flowering. 



Underground Stems. A rhizome is a creeping underground stem, 

 more or less scaly, sending off roots from its lower surface and stems 

 from its upper. The rhizome grows horizontally, vertically or ob- 

 liquely, bearing a terminal bud at its tip. Its upper surface is 

 marked with the scars of the bases of aerial stems of previous years. 

 Examples: Triticum, Rhubarb, etc. 



The tuber is a short and excessively thickened underground stem, 

 borne usually at the end of a slender, creeping branch, and having 

 numerous eyes or buds. Example: Tubers of the Potato. 



The corm is an underground stem excessively thickened and solid 

 and characterized by the production of buds from the center of the 

 upper surface and rootlets from the lower surface. Examples: 

 Colchicum, Jack-in-the-Pulpit, etc. 



A bulb is a very short and scaly stem, producing roots from the 

 lower face and leaves and flower from the upper. 



Tunicated bulbs are completely covered by broad scales which 

 form concentric coatings. Examples: Onion, Squill, Daffodil. 



Scaly bulbs have narrow imbricated scales, the outer ones not en- 

 closing the inner. Example: Lily. 



Tubers and corms are annual. Bulbs and Rhizomes are perennial. 



Exogenous and Endogenous Stems. Exogenous stems are typical 

 of Gymnosperms and Dicotyledons and can increase materially in 

 thickness due to presence of a cambium. Such stems show differen- 

 tiation into an outer or cortical region and an inner or central cylinder 

 region. 



Endogenous stems are typical of most Monocotyledons and cannot 

 increase materially in thickness due to absence of cambium. The 

 limited increase in diameter that does take place is due to the en- 



