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176 BOTANICAL GAZETTE [September 



HALODULE 



I have not examined material of this genus, but Sauvageau's 

 account of the stem structure (24) leaves little doubt that it is derived 

 from Cvmodocea or one of its near ancestors. Instead of the arc of 

 similar cortical bundles on each side of the central cvlinder, Halodule 

 shows one larger strand and a variable number of rudimentary strands 

 forming an arc — apparently a more differentiated condition than that 

 found in Cymodocea. 



ZANNICHELLIA PALUSTRIS 



The external morpholog}^ bears a general resemblance to that of 

 Potamogeton, and the minor differences are stated in detail in 

 Irmisch's classic work already referred to (12). The structure of 

 the creeping and ascending stems is very similar, and carries the reduc- 

 tion observed in Ruppia a step farther. As stated by Campbell (3) 

 and confirmed by my observ-ations, the central cylinder of the stem 

 consists of thin-walled tissue inclosed in a distinct endodermis and 

 surrounding a central cavity. The lacunar cortex lacks the two 

 strands seen in Ruppia, for in Zannichellia the leaf contains a single 

 trace which enters the central cylinder directly. Spiral tracheids are 

 present at the nodes to nearly the same extent as in Ruppia, and are 

 much more abundant than in Naias flexilis or A'^. marina, Camp- 

 bell makes the interesting observ^ation that the filament of the sta- 

 men is traversed by tracheary tissue, showing in section usually two 

 tracheids. The plant is totally submersed, and pollination is effected 

 under water, a fact which points to a high degree of specialization. 

 The single stamen and monoecious habit point in the same direc- 

 tion; in fact specialization and reduction are so great that the affinities 

 of this monotypic genus are very uncertain. 



Naiadaceae 



NAL\S 



On account of the simplicity and the exceptional development of 

 its floral parts this genus has attracted much attention. Originally 

 counted as belonging to Potamogeton, the tendency now^ is to put the 

 genus in a family by itself. Magnus (15, 16, 18) has thrown much 

 light on the morphology, and in his paper of 1870 gives an excellent 



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