74 A. H. Graves, 



female flowers, but more often the latter. From these, branches 

 of the third order arise, and in this manner a complex sympodial 

 development is the rule. 



Irmisch (1858) found that the vegetative branching of Potamogeton 

 lucens, natans, crispus, obtusifolius, and pectinatus was purely sym- 

 podial as to the rootstock, just as it is in Zannichellia and 

 Althenia. It is very probable that the other Potamogetons have 

 the same system. As to the floral system the branching is sym- 

 podial. 



Summar}'. The branching of Ruppia is of two main types : — a 

 sympodial system occurring in the region of the flowers, and a 

 monopodial system present in all other parts of the plant. 



Zannichellia, Althenia, and Potamogeton have, however, a sym- 

 podial system in the case of the rootstock, but a purely monopodial 

 development like that of Ruppia occurs in Cymodocea, Phyllospadix, 

 and Zostera. With the exception of Zannichellia, where Campbell 

 finds a true dichotomy, all these genera have a similar inflorescent 

 branch system — a single or double sympodium formed from branches 

 in the axils of the two characteristic subfloral leaves, or at any rate 

 from a lateral bud as in Cymodocea. 



C. Anatomical Structure 

 1. Growing Point. 



In a good median section the growing point of the stem reveals 

 the three divisions of primary tissue more or less clearly marked : 

 the whole is covered by a layer of dermatogen ; beneath this lies 

 the periblem, composed of usually one layer and surrounding the 

 three or four layers of plerome cells. 



Text-fig. 2, besides demonstrating these meristematic divisions, 

 shows an interesting very early stage in the development of the 

 youngest leaf. Here the periblem on the right has undergone 

 several divisions preparatory to the formation of the youngest leaf 

 primordium, which is destined to appear at this region, opposite 

 the next 5^oungest leaf, L ^^. 



On the whole, the arrangement of cells and young lateral or- 

 gans is very similar to that figured by Douliot (1890) for Cym- 

 odocea aequorea. 



