THE PLANT BODY O 



of lateral structures, which are temporary. (In the lepidodendrids, 

 Pleuromeia, and Isoetes, the functional counterpart of the root system 

 of seed plants is the rhizomorph, which, without true root structure, 

 bears adventitious, temporary roots.) Continuity of shoot and root, with 

 a transition region, the hypocotyl, is characteristics of the embryos of 

 dicotyledons and most of the monocotyledons. In some of the arbore- 

 scent monocotyledons, the embryo has no root or has only an abortive 

 structure at the end of the stem. Absence of a radicle has been con- 

 sidered by some morphologists the primitive condition in monocotyle- 

 dons, but the presence of briefly functioning and nonfunctioning pri- 

 mary roots in some monocotyledonous taxa is evidence that the taproot 

 has been lost in these highly specialized taxa and adventitious roots on 

 the hypocotyl and stembase have formed a root system. The arborescent 

 shoot system in monocotyledons is a specialized type and is accompanied 

 by a specialized, "secondary" root system. 



The Shoot 



Recent studies of apical meristems and the anatomy of the node have 

 shown that stem and leaf are closely interrelated and have emphasized 

 the concept of the shoot as, morphologically, a fundamental part of the 

 plant body. Anatomically, no structural line, external or internal, sepa- 

 rates stem and leaf. Ontogenetically, also, no separation can be made. 

 There is no constancy in details of meristematic origin of these organs; 

 histological limits of tunica and corpus vary even in the same plant. 

 The region of union of leaf and stem, the nodal region, is one of 

 merging tissues, cauline and foliar. Yet the node is commonly called a 

 part of the stem and the region of attachment of the leaf is termed the 

 leafbase. Tissues of leaf and stem merge in the nodal region, and limits 

 cannot be set; this region is a part of the shoot, neither leaf nor stem. 

 Though stem and leaf cannot be sharply delimited — and the leafbase 

 should not be called one of the basic parts of the leaf — stem and leaf 

 are units of body structure that must be treated as parts of the shoot. 



The term shoot is an old one, going back as far as the late nineteenth 

 century. Stem and leaf were, in early studies, called "correlative 

 parts" of the shoot, but "shoot" has long been suppressed, because of 

 the supposedly important distinction between leaf and stem. Now this 

 old, largely abandoned term has been revived and is slowly coming 

 into use, because it has been found to be useful and morphologically 

 valid. Although stem and leaf are convenient descriptive terms, neces- 

 sary for use in most fields of botanical study, they should not be con- 

 sidered primary categories of body structure. The flower, a reproductive 

 shoot, is treated in later chapters. 



The Stem. The stem, as an axis, is described as divided into parts, the 

 internodes, by more or less well-defined areas where appendages of leaf 



