l82 



BOTANICAL GAZETTE 



[SEPTEMBER 



the ventral strand. In the elongated internodes of Potamogeton only 

 collateral bundles occur, while in the shortened axis of Triglochin, to 

 which many leaves arc attached, amphivasal bundles are numerous. 

 This apparently represents the direction of evolution. Lack of mate- 

 rial has so far prevented me from studying the seedlings of any of the 

 Helobiae, but the importance of such study is fully recognized, and 

 cultures are in progress which it is hoped will shortly render it pos- 

 sible to fill this gap in the evidence - 



Admitting then the derivation of the monocotyledons from dicoty- 



ledon-like ancestors, which of the 



genera 



studied most closely 



resemble the dicotyledons ? There seems to be sufficient reason for 

 considering Potamogeton the most primitive genus met with, as is 

 indicated by the following facts : 



(i) The stem of Potamogeton is not a shortened axis, such as is 

 seen in many of the monocotyledons, e. g., the rhizome of Triglochin, 

 which in all probability has been shortened and thickened in accord- 

 ance w^ith its geophilous habit. 



(2) Xylem is well developed in the nodes, floral axis, and young 

 stem of many species of Potamogeton. Its presence in these regions 

 cannot be explained on physiological grounds, but must be regarded 

 as the persistence of an ancestral feature. Conduction must be per- 

 formed by the internodes as well as the nodes, therefore if the trach- 

 eary tissue wxre functional (i.e., rudimentary instead of vestigial) in 

 the immersed species, its occurrence ought not to be localized. 



(3) The separate strands present in the central cylinder of such 

 species as P. piilcher are to be regarded as more primitive than the 

 compound or fused cylinder seen in Zostera, etc. 



(4) Potamogeton shows the circle of bundles in the floral axis 

 more plainly than any other genus studied, w^ith the exception of 

 Triglochin. The significance of this feature has already been con- 

 sidered ^ 



(5) The phloem in Potamogeton is particularly well developed, 

 consisting of sieve tubes with evident sieve plates, and undoubted 

 companion cells, though the latter are seldom so plain in monocoty- 

 ledons as in dicotvledons. 



(6) Some species of Potamogeton possess floating leaves. Though 

 it is not denied that this may be a specialized feature, it has been well 



