8 J. O. HAGSTEOM, CRITICAL RESEARCHES ON THE POTAMOGETONS. 



4) interlacunar bark-parenchyma and 5) its occasional strands, 6) endodermis and 

 7) the central cylinder with its vascular bundles and channels, the latter originating 

 in a more or less dissolved xylem, and eventual mechanical system. 



The epidermis consists sometimes of small cells sometimes of wide ones, round- 

 ed — squarish — oval in crosscut-form, usually many times longer than their width, 

 but at times, to facilitate their mechanical function, much abbreviated almost cubic, 

 for instance in the P. vaginatus, fig. 12, D. 



Thehypoderma (pseudo-hyp.) sometimes occurs one-celled, sometimes 2 — 4-celled, 

 see fig. 116, D, occasionally it is wholly wanting. In this case the very epidermis-cells are 

 usually bigger, their crosscut-form oval with the radial axis longer than the tangential. 



Subepidermally occurs often — though it be as frequently' lacking — a more or 

 less complete circle of more or less strong bastbundles situated in the angles where 

 lacunar septse join the epidermis or its corroborating stratum. 



The cortical lacunae are mostly widest in the subepidermal circle, but occasio- 

 nally their greatest widening appears in the circle next to it. As the lacunae 

 of each circle have almost the same width or lumen, the cortical vascular- and 

 bast-bundles also show a circular arrangement (in one or a few more or less com- 

 plete circles). 



Owing to the fact that the outer circle of those interlacunar bundles originate 

 in the bundles of the sheaths and ligules (see the fig. 88, vbic) this circle should be 

 named the ligular circle. It is natural that the ligular circle is composed by a greater 

 or less number of strands at different levels of the stem as the sheaths and ligules 

 are of different thickness and length according to their place on the stem. The 

 Coleogeton-STjiecies are visually provided with stout basal sheaths. 



According to the above said you will find the most advanced, constant and 

 typical anatomical conditions prevailing at about the middle of the spike-bearing 

 shoot, from which level thus the crosscut for systematic purpose is to be taken. 



The anatomy of the rhizome is rather uniform throughout the whole genus. 

 The difference chiefly pertains to the character of sclerenchymatous or non-scleren- 

 chymatous species and the degree of coalescence of the central vascular bundles. 

 The form of the rhizome is also nearly the same, namely terete or at least nearly 

 so even in species with compressed or bicanaliculate stem. In order to show 

 that interesting fact I have figured cross-sections of P. crispus and P. acutifolius, 

 see figs. 22, A and 26, B\ 



As to the peduncle there is an essential difference between the CoZeog'etow-species 

 and the species of the subgenus Eupotamogeton in the former being furnished with 

 an endodermis, the latter, again, lacking this organ. On P. trichoides and joliosus, 

 vide the descriptions! Within the Eupotamogeton-ST^ecies the peduncles sometimes 

 offer differences in form, in general, and crosscut-form, but also as to some anatomi- 

 cal facts, for instance the absence or presence of subepidermal strands in some groups 

 especially in the Pusilli. The arrangement of the bundles is also noteworthy in many 

 species. The more or less prominent stiffness of the peduncles of the Eupot.-&^ecies 

 is caused by the bundles spreading and becoming surrounded by strong bast sheaths. 



