CHAPTER III 



THE ANATOMY OF THE SHOOT 



GENERAL MORPHOLOGY OF THE STEM 



THE stem and its appendages, foliage leaves and inflorescences, con- 

 stitute the shoot. Tlie stem is typically an aerial structure; 

 but, in some cases, it may be wholly or in part subterranean, pro- 

 ducing underground branches some of which may be greatly spe- 

 cialized for storage. 



In general form, the aerial stem may be erect and unbranched, 

 bearing the leaves and flowers on a single vertical axis, or much 

 branched with several axes of approximately equal size so that it 

 develops a bushy crown. In a vine type, the stem may be prostrate 

 as in Ipomoea, twining as in species of Phaseolus, or climbing by 

 means of tendrils, Pisum and Cucurbita. In some plants, the stem 

 does not elongate greatly during its early development, and forms 

 a short conical crown of leaves. This type of growth may result 

 in the formation of a bulb if the leaves are fleshy (Allium); a head 

 (some species of Brassica and Lactuca), or a simple rosette (Beta 

 and Raphanus). Later, the stem or a portion of it may elongate 

 rapidly, forming an erect axis bearing foliage leaves and the 

 inflorescence. 



The principal types of underground stems are the rhizpme, tuber, 

 and corm. The rhizome consists of a more or less elongated under- 

 ground stem which arises from a lateral bud near the base of the 

 main stem axis and extends horizontally through the soil. Al- 

 though it may have a superficial resemblance to a root, it has the 

 anatomical characters of a stem with respect to the internal organi- 

 zation of its tissues. Externally, it may be recognized as such by 

 its nodes and internodes, although the underground habit results 

 in the reduction of the leaves diverged from it so that they are 

 small and scale-like. Functionally, such stems are often storage 

 organs, and a means of vegetative extension. Aerial shoots fre- 

 quently develop from lateral buds of the rhizome, and adventitious 



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