1 2 MEMOIRS FROM THE DEPARTMENT OF BOTANY OF COLUMBIA COLLEGE. 



THE ANATOMY OF THE STEM OF POLYGONUM. 



The genus Polygonum is made up of a number of natural groups of plants which are 

 closety related from a morphological standpoint. These groups or subgenera, as they are 

 now technicall}^ called, each include a number of species which agree with each other in 

 hal)it and appearance, and although thus clearly separated from one another, they mor- 

 phologically form a distinct and natural genus. 



Polygonum as represented throughout the world is divided into eleven subgenera. 

 Nine of these occur in America, and eight, namely, Bistorta, Aconogonon, Persicaria, 

 Tovara, Avicularia, Duravia, Tiniaria and Echinocaulon are indigenous. Although there 

 is more or less variation as regards the separate elements of these sections, the sections as 

 a Avhole show marked characteristics, especially in habit and habitat. 



Bistorta and Aconogonon are either alpine or sub-alpine and grow erect. Two other 

 sections, Persicaria and Tovara, are never normally alpine, they prefer and flourish best 

 in damp situations, their position is usually erect and their aspect not dissimilar. Sec- 

 tions Avicularia and Duravia thrive best, as a rule, in dry places, while Tiniaria is 

 scandent by its twining stems, and Echinocaulon reclines on surrounding objects and 

 often climbs by means of recurved prickles. 



These general variations in habit and habitat suggested a comparative study of the 

 stems of the eight indigenous sections. For this purpose a type has been selected from 

 each, in most cases the type species, if it is native in America ; otherwise a near relative 

 was taken. For Bistorta, Polygonum bistortoides ; for Aconogonon, P. alpinum; for Per- 

 sicaria, P. Pennsylvanicum ; for Tovaria, P. Virginianum ; for Avicularia, P. aviculare; for 

 DuraA'ia, P. Calif ornicum ; for Tiniaria, P. scandens, and for Echinocaulon, P.. arifolium. 



The anatomy of the stem of Polygonum, as brought out by the eight examples, will 

 be described below, and this general description will be followed by a detailed and com- 

 parative account of the structure of each type. 



THE GENERAL ANATOMY. 



The stems under consideration are composed of three cylinders which are derived 

 from the three cell-layers of the embryonic or meristematic stage. The first, that arising 

 from the dermatogen layer, is the epidermis; the second, that following the epidermis and 

 derived from the periblem layer, we will i design ate as the periblematic cylinder, while the 

 third and largest, which is produced by the plerome, forms the pleromatic cylinder. 



These cylinders, in their mature state, are well developed in all cases, clearly marked 

 off from each other, and are also respectively composed of the same tissues and elements. 



