164 CLASSIFICATION OF ROOTS AND STEMS 



ing, the flattened surfaces usually looking upward and downward. The 

 presence or absence of branches is always characteristic. The manner 

 in which the roots take their origin is c(|ually so. These may form a 

 circle (Fig. 447) or be restricted to the under surface (Fig. 449). The 

 number of roots developing from a node is usually fairly characteristic. 

 So is the persistency or brittleness of these, and the characters of the 

 stumps or scars which they leave, as well as their form, which is very 

 often triangular or quadrangular in section. Their structure, as observed 

 either with the lens or with the microscope, is characteristic and of 

 diagnostic value. Sometimes the roots are not only restricted to a 

 certain portion of the node, but in the case of short rhizomes are re- 

 stricted to a definite portion of the latter (Fig. 451). The relative length 

 of the internodes of a rhizome, as compared with its diameter or thick- 

 ness, calls for close attention, and so does the absolute or measured 

 length. The relations of the erect portions to the horizontal, and the 

 stumps or scars left by the former upon their death or separation, con- 

 stitute one of their most important diagnostic characteristics. Com- 

 monly, disarticulation occurs, with the production of a cup-shaped scar. 

 This scar will be characteristic as to whether it form a depression in the 

 general surface (Fig. 450) or be elevated upon a base (Fig. 448), as will 

 the length of the latter, the form and depth of the scar, and the char- 

 acter of its edge. The size of the scar, that is, its lateral breadth as 

 compared with the thickness of the internode, is also noteworthy. 

 Leaf scars, or leaf remains, upon rhizomes call for the same examination 

 as do the stem scars. They may surround the entire rhizome, in which 

 case they are designated Annular, or they may be confined to the upper 

 surface. If the latter, the scar may be of characteristic form, as linear, 

 elliptical, circular, cordate, crescent-shaped, or ^"-shaped (Fig. 452, a). 

 Finally, we note that annular or longitudinal folds, thickenings, wrinkles, 

 or constrictions are characteristic of certain rhizomes as well as of roots, 

 particularly in the dried state. 



Mode of Extension. — Simple and Branched Stems. — As to their mode of 

 extension, stems may be Simple or Branched. A stem denominated as 

 simple is not necessarily entirely destitute of branches, as floral branches 

 or small branches near the summit are permitted. It has already 

 been shown that stems may develop monopodially or sympodially. 

 The stem of a tree which continues, except in case of accident, to develop 

 monopodially, as the Fir, is called Excurrent. One which after a time 

 loses its main stem in a number of branches, as for instance the elm, is 

 Deliquescent. 



