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. 
Summary. 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 youngest leaf, LZ . 
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. 

